How to control power from a dc bus to a hbridge?

How would I control the flow of power from a DC bus to a H-bridge with an arduino uno? I don't want to generate power from the arduino to drive the motors.
In my mind I would like the arduino to control a varistor with a robot arm, but I know there must be a better way.

I don't have the exact specs on the h-bridge but I have an existing RC curcuit that works to control it - and I have a multimeter and have started to take voltage-drop measurements while operating and not operating for comparison with the curcuit I want to build. I am guessing that If I can create the same voltages than I can get the motors to run under arduino control (but I may be missing something - my software skills are much better than my hardware skills).

Also how do I determine the exact model and firmware/hardware version of my arduino uno. I think that may matter.

What DC bus?
Controlling power leaving a connection is done by switching loads on and off.

6 x 1.5 volt batteries external to the arduino
Could the power be modulated on and off across time then smoothed out with something like a capacitor to emulate a varistor?

Arduinos don't transmitt power. You need a driver to do that.
PWM form the UNO to the driver and then filtering the driver output is a possible theory.

Would I buy the driver as a module or make one with a transistor? Pulse-width modulation (PWM) sounds like what I want. This diagram ( https://images.prismic.io/circuito/8e3a980f0f964cc539b4cbbba2654bb660db6f52_arduino-uno-pinout-diagram.png?auto=compress,format ) looks like it is saying the arduio can produce 5v or 3.3v against grnd - which I can imagine providing the reference / control voltage/current for a transistor, while the 6 x 1.5 batteries are the controlled voltage/current. I am not sure I am using the right words. Does PWM produce 5v or 3.3v for each pulse? I imagine that the transistor would work like a resistor and would heat up when current is flowing through it - I don't know if there is some kind of transistor specification that tells how much heat/current it can take under normal usage. I also don't know exactly how to design the control circuit for the transistor where it connects to the arduino. I guess if I don't want to burn the arduino out I should have a resister in the control loop, but I have no idea how to chose the value of the resistor. I don't know if I should try to condition the pulses into a not-all, not-none voltage before the transistor (in the control loop) or after (in the controlled loop). I only know the basics. Does the transistor introduce a voltage drop itself or can is be viewed as transparent like a switch?

I don't find that link interesting only showing the pinout.
A 5 volt Arduino produces 5 volt PWM. A 3.3 volt Arduino produces 3.3 volt PWM.

Make a kind of wiring/logic block telling what You want to control.
What kind of load will You use? How much current will that load need?

You must be specific. Talking in general words will never give You any precise answers, only a lot of heavy theory and more things You must know.

Thanks for all your help. You said "A 5 volt Arduino produces 5 volt PWM. A 3.3 volt Arduino produces 3.3 volt PWM." How do I know which one I have?

I don't have the exact specs on the h-bridge but I assume that it looks like this on the inside:

I am still learning about h-bridge designs:

https://www.modularcircuits.com/blog/articles/h-bridge-secrets/h-bridges-the-basics/

I am trying to figure out where to probe the voltage with my voltmeter to get a better idea of what the circuit is like on the inside.

This looks relevant too

https://www.modularcircuits.com/blog/articles/h-bridge-secrets/h-bridge_drivers/

As does this:

You said you have an Uno, this is a 5V board.

BTW you confused a varistor with a variable resistor - not the same thing.

I hope not, that's got shoot-through - with H-bridges you have to switch off before switching on, that circuit switches on before switching off, leading to high current spikes during switching (whereas there should be some deadtime).

That circuit lacks free-wheeling diodes too, so it will probably fry due to inductive kickback.

Again, what kind of load will You use? How much current will that load need? Voltage is 9 volt....

The load, the need for current, voltage used, set the demands for the driver.

Does the load need changed polarity like a motor running both CW and CCW?
Is it a passive load?

Those questions strongly affect the type and size of the driver.
For a passive load a single logic level N channel MOSFET transistor could be the solution.

The application is a small robot. The load will be DC motors. I am modifying something I already own so I don't have all the specs for the components. Suppose I wanted to measure the load with a volt meter or a multimeter, how would I do it?
I can try to work up as much of the circuit diagram as I can with a pencil and paper. I could even transcribe it to Inkscape. But there are a lot of pieces I have no specs on.
My understanding is that the DC motors will draw variably based on the surface conditions - such as rug or tile, but I can get ball-parks with a multimeter if that makes any sense. If the rover hits a wall the resistance will shoot up. It least that what I think.
I could set up a number of experiments for different conditions. The current set up uses RC and I want to convert to Arduino control and take out the RC module.

This seems to be part of what I am looking for:

No, that's unfortunately one of the many un-curated bad circuits on instructibles. Building your own H-bridge is more prone to issues than you might think.

Although the circuit might seem to work, the transistors on the top-side will get very hot or burn, the transistor type specified (BC547) are only rated for 300mA (less than most small motors need), the base resistor values are plain wrong, and 9V battery is an inadequate source of power for a motor (unless its one of the new fangled LiPo versions, but they are only 7V anyway).

Okey. That changes things a lot. Some trial and error is unavoidable.
Controlling motors running both directions a H bridge is needed.

Go for MOSFET bridges as they waist less of voltage. I guess Your motors likely use a few amps at the most.
Yes, sure the mechanical load on the motor affects the current a lot.
As those bridges are fed with PWM voltage measuring is not useful.
A shunt for current measurements and an oscilloscope would manage. I think You can proceed without that. Keep watching temperature on the drivers/bridges. They don't blow up like Chernobyl.

I haven't been a electronics class room in years. Everything we did back then was much simpler than this. If transistors are rated for some number of amps and amps is flow rate - voltage is more like water pressure, and the heat in the transistor will be - um - power I think? watts? V = IR and P = I V? So for a given voltage - does the transistor have an internal resistance which would be documented? Then I could calculate the current and the power. Or do I assume it is a conductor (or equivalent). If I measured the voltage drop I could calculate the internal resistance. I am in a bit over my head. I am not sure what questions I should be asking myself and where to start measuring. Do I want the voltage drop across the motors or the drop before the h-bridge across the RC module?

Here is the diagram with as much detail as I have.
For more detail I have to take measurements.
I want to replace the "R/C/ Receiver" with the Arduino Uno.
I think the "Motor Controller" is just an H-Bridge (or two packaged together) - but I am not sure.

M1 drives the two left wheels (which are mechanically linked together).
M2 drives the two right wheels.
M1 and M2 can be turned on independently to execute a turn, both driven at the same time for forward or reverse or both driven in opposite directions to execute an about face. The RC controller has two joy sticks which can really only be in forward or reverse position (each) but which work independent of each other and two "fire buttons" which control the LED (the only diode in the picture) or the "Horn".I am not really sure how the Capacitors fit in, but I assume they have something to do with RF modulation/demodulation - so they can probably be removed when the arduino is in place.