Before every deploy your branch has master merged into it. There’s some clever work by Hubot while you’re in line to deploy to create a version of your branch that has the potential new master / main branch. If conflicts arise you fix them before it’s your turn to deploy.

how deep does this deploy queue get? And does everything that lands on master have to first be deployed?

This seems like a bit tough for getting work done if you have enough people all waiting around for this

The deploy queue usually gets to be a couple of "trains" deep which usually includes work from 4-10 devs. This represents a couple of hours. We have had issues with it taking too long, but this work I wrote about has improved that! We continue to try to improve it.