How do app updates work if the system partition is read only? I have a Android phone (Samsung). It comes with a whole bunch of pre-installed stock apps.

Q1: Is it correct that these stock apps are stored in the read-only partition /system?

Since getting the phone, I have updated most (if not all) of the stock apps via the Play Store.

Q2: Assuming app updates can't touch /system, how do they work? How can apps be updated if they are effectively read-only?
A1: I'm pretty positive that they are on the /system partition because you can't delete them, if you could, that would mean they are in /data partition or whatever it is called where the userspace is.

A2: I think they install updates in userspace, not in /system. The reason for believeing so is that after you update a system app, and you go in Settings -> Apps, it allows you to uninstall updates only, meaning that the update that was installed in userspace will get deleted but the original app, the version you got out of the box, is still left untact in /system, and you can't delete it.