> How can "sudo apt install chromium-browser" not install the apt package but install instead a snap?

Simple: they decided that maintaining the chromium .deb takes too much effort, and that snap is making that easier. So they moved it to Snap.

But there are many users out there with existing installations who have selected chromium as one of their installed packages. And so as not to keep said users without updates, they decided to migrate them over the best way they found.

I'm not completely sold on the snap ecosystem because of some technical issues, as well as the issues of fully automatic updates. But this particular question has been given a lot more air than it deserves.