Set up for a 6v-8.4v 40KG servo

Hello I am a total Noob so any advice would be great. I would like to keep this as simple as possible.
I would like the whole project to be powered from transformers and not batteries.
I am planning to power the Arduino from a separate 12v power supply into the jack just to keep things simple and to avoid any brownout problems.

For the servo this is my thinking.
Data line going from pin 9 straight to the servo

I am assuming getting a powerful 7.4v transformer for the servo is going to be problematic so I am thinking I will need a 5A 12v transformer with a 5a buck to bring the voltage down to 7.4v
This will then power the servo.

I am unsure of this part.
Am I correct in thinking that ground from the servo will also have to go to the ground on the Arduino? Is ground always 0v? I'm just worried that 7.4v from the buck will connect to the motor and then connect to the ground pin on the Arduino and I am worried the 7.4v will damage the Arduino as a lot of projects usually use 5v.
If the 7.4v is a problem for the ground pin do I need to use a resistor between the servo and the ground pin? or some kind of shield?

Am I thinking alone the correct line.

Controllers and servo need DC. Transformers delivers AC.

Was thinking something like this
inShareplus 12V 5A 60W LED Power Supply, 100-240V AC to 12V 5Amp DC Power Adapter, Low Voltage Transformer for LED Strip Light https://amzn.eu/d/66xd0Fk

I think this is dc output.

That is the best way to destroy an Arduino. The supply jack goes through the on board 5V voltage regulator. This can only handle about 200mA. It can not handle the current required to drive your motors.

Yes.

No. Ground can refer to the actual ground, that is a metal stake in damp soil.
Or it can refer to a common reference point. This is normally 0V but can be other voltages. Like a car can have a positive ground or negative ground.

No.

That is quite some buck converter. Check to see if you can buy / afford one.

Why is it, that as a total Noob, you want to do a complex project first? It is a mistake that is often made.

I was thinking something like this for the buck converter.
https://amzn.eu/d/hq3hckR

Re the separate power supply for the arduino. I thought the ampage doesn't really matter as long as it is above the requirements of the arduino.
Doesn't the Arduino only draw the ampage needed so if the transformer has a high ampage it doesn't really make much difference.
You do mention that a 12v power supply will go through a 5v voltage regulator. With that in mind is a 5v power adapter much better for powering the arduino?

Thanks guys for all your hints and advice. It is appreciated.

I didn't actually think this was a complex project. Am I mistaken?
1 power for arduino
1 separate power for servo
And data.

Read the reviews on this one, especially the critical ones for example:-

1.0 out of 5 stars Product fails after 2W of Use
Reviewed in the United Kingdom on 25 November 2020
Verified Purchase
Used these in a ham radio power supply and used well within specification. Product failed after a couple of weeks of use. Fortunately this was a two pack and the other has failed too. Seems no mechanism to get a replacement or refund as I am outside of the 4W period. Bottom line - can't recommend.
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Which is what I would expect from this product just by looking at it.

It matters if you expect to drive anything from the 5V pin of the Arduino. So as long as you are not trying to drive the servos from 5V or anything heavy you are OK.

Yes that is why I said so.

Anything that involves unusually high current requirements, when you know little about what you are doing it too complex for a beginner.

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