Multiple Automations with the same trigger or one long automation

I think that if you have multiple automations with the same trigger, that they will both run at the same time (or nearly the same time). As in they dont have to wait for one to complete before the other can start.

I have a few fairly long automations for when I go to bed, and when everyone leaves. I was thinking that maybe it would be faster to create 4 or 5 automations with the same trigger. Like one to lock doors, one for the lights, one to arm the security system, etc?

First, am I right as in they would all trigger at the same time, and how does anyone else do it this way

For all practical purposes, yes.

For ease of debugging I generally try to keep one automation per trigger or list of triggers.

It also lessens the possibility of race conditions. e.g. If you have two automations that trigger on the same event and do different actions but those actions cause further events and the order the subsequent events occur is important.