There is no apt package with chromium, but there is an apt package name 'chromium-browser'.

https://packages.ubuntu.com/focal/chromium-browser

In previous versions of ubuntu, that package would install chromium, as one would expect.

In recent versions of ubuntu, this package is a stub that installs snapd, and then snapd installs chromium.

There is a substantial difference between those two things. Users that try to install chromium through an open package management system are being pushed to Canonical's proprietary store.