I have a quick question:
I am wondering if it is possible to connect all the grounds together and make one "universal ground" (aka just add a copper layer at the end which connects all the grounds).
I have attached the schematic I have bellow. My concern is that I have a lot of capacitors which connect 5V to ground (like C6) and I am worried that by connecting all the grounds I will end up shorting the circuit or the connection will not be made.
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Thanks!
Brand new interactions and interfaces. 
Easy to use and quick to get started. 
Usually you can connect just about everything to the ground plane. However you possibly have the two of the four cases where you don't - r.f. and high current switching. (the other two are audio and SMPSes).
Firstly C15 and C16 need a guard band arrangement. See
https://www.st.com/resource/en/application_note/cd00221665.pdf&usg=AOvVaw3ZeLhVSPbV6Bx72UWMxyCi
on how best to do this.
Also the whole NRF24L01 area probably needs its own ground plane taken back separately to the main ground star point at the input connection otherwise you'll possibly connect MCU noise onto the antenna. I expect there is an example layout in the application note which I suggest you copy exactly assuming it includes the guard band mentioned above. If it doesn't, it's a crap application note :-)
You also have the magic word 'motor' which could imply high switching currents. If so then the VCC and GND for these need taking back separately right to the battery connection, i.e. the current for U2 mustn't come through the same path.
An easy way to achieve all of this is to place the battery connector in the centre of the longest edge and place the motors to one side, the r.f. to the other, and the MCU and regulator directly below. If you do this, then you may in fact be ok having a single ground plane.