9 times out of 10 the software you’re looking will typically land in your Distribution’s repository, before it lands in the main repository it’ll be vetted for stability and security in a testing repository.
For example; Steam-Installer is located in the main repository for Debian 12 (Bookworm) they also have a newer version in their Debian 13 (Trixie) repository for testing the next generation of Debian..
If you want to install software outside your distributions repository you will need to vet the software yourself and make sure it’s compatible with your distro.
Hope that explains it a little easier.
This explains SO MUCH! I was getting frustrated when games start out perfectly fine than 30 minutes in frames would drop significantly.