I've been using Arch for about 15 years or so, and yes, I build up cruft... in my home directory ;-). The system itself is remarkably good at keeping tidy. The one spot to keep an eye on is /var/cache/pacman, as that's where it stores every package you download before installation and it won't delete it without you asking it to.

Any new config file will be saved with a .pacsave extension, so you'll want to keep an eye out for those, but that's basically it

Not in any bothersome way. But if you really want to reinstall often that is valid as well. You can very easily script the arch install process to get you back to the same state far easier than other distros as well. Or you can just mass install everything except base and some core packages and reinstall the things you care about again which almost gives you a fresh install minus any unmanaged files (which are mostly in home and likely want to keep anyway).

The main trash you accumulate are config files in you home directory because they stay after the package is uninstalled. And they just sit there not hurting anybody.

Most of the junk accumulates in /home and I did a cleaning once, where I got rid of a couple hundred GB there, from stuff that was either already uninstalled or still installed but unused for years.

In the other root directories, I didn't find much tbh. My / (excluding home) takes up 40GB and I don't think it was significantly lower years ago as the bulk of it comes from necessary program files.