There are a ton of interesting apps to use on Linux.
The Steam Deck uses Flatpaks from Flathub by default in KDE's Discover app "store".
Browse and search https://flathub.org/ to find all sorts of apps.
Are there any types of apps you're interested in or anything in particular that you're used to and want to find something similar?
You'll probably want Flatseal to adjust Flatpak app permissions (like giving apps access to more directories and such). You usually don't need this, but some apps might not support portals and may need more "holes" poked in the sandbox to access additional directories.
If you want to run VMs, then I'd suggest GNOME Boxes (just searching for "boxes" in Discover should find it. You could install a standard Linux distro inside — or even Windows if you really wanted. It's simple and straightforward, and even has Linux distro downloads from the UI (so you don't even have to hunt for an ISO to download).
If you're looking for command line stuff, then I'd suggest setting up podman and distrobox; this is more advanced, but would let you run almost everything from any distro on your Deck via containers that persist (even across Steam Deck updates): https://github.com/89luca89/distrobox/blob/main/docs/compatibility.md#install-podman-in-a-static-manner & https://www.gamingonlinux.com/2022/09/distrobox-can-open-up-the-steam-deck-to-a-whole-new-world/
If you're doing a lot of stuff on your Steam Deck in desktop mode, please remember that it's not encrypted (so anyone can copy anything you do out of it) and doesn't even have a password by default. You'd want to set up KDE Plasma's Vault and use that for sensitive documents and/or use a VM with disk encryption. Sadly, EncFS isn't included on the Deck, so you cannot use KDE Plasma Vault (without jumping through hoops), so you'd have to set up something similar in Distrobox (although probably a different program), use something similar from Flathub (again, which requires a command line program behind the scenes currently) or use a VM with encryption (which is the easiest thing to do).
If you aren't doing anything sensitive (including browsing) on desktop mode and don't want to do anything too advanced (like command line stuff), then all the many thousands of apps available from Flathub are still really astoundingly great and super easy to use. They're basically just a click away in Discover.