There are only few apps on appcenter. I can't install eddy to install .deb packages. I tried installing a flatpak app and it didn't work. I saw a few users asking about the same.
If you sideload a flatpak from flathub.org you'll automatically add their repo, and populate Appcenter a bit more.
Other than that it seems AppCenter won't support deb packages anymore. So yes, you can still install deb programs via the terminal, but I'm afraid you'll then need to manually update them, because appcenter won't do it for you.
The reasoning for this apparently is that some users found non curated apps "ugly" so now they castrated the whole os for it.
Eddy now are in AppCenter DON'T INSTALL FROM GITHUB
I was able to install Eddy from the terminal like this:
sudo apt-get install git sudo apt install valac libgranite-dev libpackagekit-glib2-dev libunity-dev sudo apt install meson git clone https://github.com/donadigo/eddy.git sudo apt install libzeitgeist-2.0-dev meson build && cd build meson configure -Dprefix=/usr ninja sudo ninja install
and to run the application:
If this solves your question don't forget to mark as solved