Can a motherboard be a simple circuit between arduino uno and a set of devices (ie: motors)
I would call it an &quot;interface&quot; or &quot;driver&quot; board. You don't need to get too hung up on the exact name.
The word &quot;shield&quot; is one of the Arduino-peculiar words (like &quot;sketch&quot; for &quot;code&quot;) which now has a generally-understood meaning. That meaning is, something that plugs directly onto the Arduino board, with the pins on the shield matching the sockets on the Arduino board.
Whilst a shield is a quick and easy way of getting started (because you don't need to wire anything) they can have their disadvantages. One being, what if you want two different functions, like a motor driver and an Ethernet interface? Two shields, one on top of the other? What if they both use a certain pin for different purposes?
I made up a radio-controlled car, see photo:

The only shield I used was a generic &quot;put whatever you want onto it&quot; shield, which let me mount some MOSFETs, a radio receiver board, and other components, as required by the design. There is a Uno underneath it, not particularly easy to see. Adjacent on the right is a motor driver board. A LiPo battery is slung underneath.
This kept things reasonably compact, without requiring a specific purpose-made shield which only does one thing.

and a custom circuit board (hence my question) will act as the interface between the uno and the motors, providing electricity from a dedicated source to the motors.

It sounds like your project is similar to my car. Just assemble things in a way that makes sense to you.