Firstly you are talking about amplification, not isolation.
Isolation means two circuits have infinite resistance between them, and is accomplished
with a transformer or opto-isolator.
Here we are boosting logic signals to many amps to power motors - this is simply
amplification (aka buffering).
The L293D is a darlington output H-bridge chip capable of upto about 1A
without efficient heatsinking. It takes logic signals in and a 5V supply on
one side, and controls a motor fed from a motor supply on the other. grounds
are common (no isolation here).