Very interested in vanillaos for server and desktop - questions

Initially this got my attention for my home server. I liked the idea if immutable but not the locked down feeling and restrictions on software. So vanillaos apx installer got my attention.

My main question is regarding updates of the packages installed with apx. Say I installed steam via the arch repos package, using apx. What drives the updates for this package? When apx update (or whatever the command) is run, ​​does it check that packages source for updates? So it would check the AUR for updates? If another package is installed via apt repo, will apx check that repo in the future for updates for that package?

One last question; take brave browser on Debian stable for example. ​I would go to Braves website and add the repo from their website command (curl command) to install brave via their repo as it is more up to date than Debians apt repo. ​Is something like this possible on vanillaos?

For both of those questions, I'm only interested in answers that don't compromise the immutable core of the system.

Thanks in advance