My friend and I are repurposing an old rc car for a project. When we connect the dc motors to 5V and ground, the motors spin. However, when we power the motors from 5v and ground it in the digital output pin set to LOW, nothing happens. The identical circuit works in tinkercad. We are using a nano, but have tried an uno and mega as well.
Never directly connect a motor to an Arduino IO pin
You will destroy the Arduino.
An Arduino cannot source (or sink in your case) enough current to drive a motor, and if the motor did manage to turn the huge amounts of EMI it generates would literally blast holes in the silicon inside the chip.
You must use a motor driver of some form. If you only want one direction that driver can be a single NPN Bipolar transistor or N-channel MOSFET. If you need it to change direction then an H-bridge is what you want.