Zombie Respawns - specifics

Ok so I understand that zombies have a respawn timer of 30 minutes, which is configurable.

My question is, when does that 30 minutes start? Is it the moment they die, or the moment their dead body despawns, or when the player is a certain distance away?

I guess I mean, if I kill a zombie with an axe 1m away from me, and kill another with a rifle 100m away from me, and then kill another with that rifle 300m away from me (all the while, I stand still in the same spot), do they all respawn at the same time, or are one or more too close to respawn? Is "too close to respawn' a thing?

I'm coming from DayZ and Project Zomboid and I'm trying to get my head around the differences.