How do helicopters avoid colliding?
Just like other aircraft flying VFR, see and avoid. Helicopters fly slower, which gives them more time to see and be seen.

They also have transponders and ADS-B requirements so it will be possible to see them electronically too (with TCAS today and ADS-B in in the near future).

While helicopters could technically have no transponder and hence no ADS-B requirement, operating inside big cities leads to the under class B veil, which ends up forcing transponder / ADS-B requirements anyway. What I really mean is there's little logic in owning a helicopter that can never go into the big city, those are usually expensive enough to warrant being able to go anywhere, unlike some old cheap piston planes.