Will the board power itself while connected to usb?
I'm wanting to power it off of a vehicle, what do i drop voltage to?
Can I use old relays from a vehicle instead of buying a solid state relay board?
Welcome.
Yes power it from USB. To power it in a vehicle, use a standard cigarette lighter cell phone charger, almost all of them nowadays have a USB connector. The relays can use the vehicle power directly.
You can use any relay. You may need to measure with a multimeter how many milliamps it takes to drive the coil of the relay, if it is not on the package or stamped on the relay. You will need a transistor, resistor, and diode for each relay you want to drive with the Arduino. Here is my favorite article about how to use a transistor:
Once you figure out what you want to do, draw a schematic and post it with a description of what you intend the circuit to do, and ask any questions you have. Some of us on the board can offer pointers and advice. Use the General Electronics board for that. Neat pencil drawn schematic is fine, you don't need to use a drawing program for that.
https://learn.sparkfun.com/tutorials/how-to-read-a-schematic