I’ve always wondered about LiDAR - how can multiple units sweep a scene at the same time (as would be the case for multiple cars driving close together, all using lidar)? One unit can’t distinguish return signals between itself and other units, can it?

Pulse modulation.

https://opg.optica.org/oe/fulltext.cfm?uri=oe-31-2-2013&id=5...

I think it can - I think it might encode something in the beam, use a slightly different wavelength or even just pulse the laser so that the returns don't overlap if timed right.