I have been away for 4 months and when I started Ubuntu again there were 0 updates available. That seems very odd to me. I tried several times and always got that the same result: packages are all up to date. How can that be? Any suggestions?
Ubuntu 20.04.3 LTS - 64-bit - Gnome Version 3.36.8
$ sudo apt update OK:1 http://archive.ubuntu.com/ubuntu focal InRelease OK:2 http://archive.canonical.com/ubuntu focal InRelease OK:3 https://deb.opera.com/opera-stable stable InRelease Paketlisten werden gelesen... Fertig Abhängigkeitsbaum wird aufgebaut. Statusinformationen werden eingelesen.... Fertig Alle Pakete sind aktuell.
focal-updates
andfocal-security
sources itself. Whatever reason you had for disabling/deleting those sources, it was unwise. Restore them, and don't do that again.