Debian GNU/Linux contains a lesser known external repository tool. It's called extrepo and it's available in the official stable repository. Extrepo doesn't contain as many packages as AUR (the external repo for Arch Linux), but it still might be useful to some people.

Just to name a few repos it contains:

• VS Codium repository (FLOSS binaries of VS Code)
• Ondřej Surý's package repo (newest PHP packages)
• Valve Corporation's Steam repo
• WineHQ repository
• VirtualBox official repository
• Brave browser release repository
• Prosody XMPP server

For a full list you can browse the config files

As root run:
apt install extrepo
Then have a look at the man pages which explain how to use it, it's very simple.
man extrepo

For example if you would like to add the VS Codium repo and install Codium, you would just run (as root)

extrepo enable vscodium
apt update
apt install codium

But don't forget that Debian's official Backports repository should be the first one to look at if you want a newer version of a particular package, compared to the stable branch. Reason: security.