Ubuntu 21 (impish) updates unavailable

I was trying to add some new packages and figured it out that my Ubuntu 21 is out of date. Unfortunately some time ago I unchecked all repositories in Software & Updates so my /etc/apt/sources.list was empty. But I managed to have it working properly adding this one:

deb impish main universe


apt update

and I was more or less back on the track. How trivial was it just to be able to install default-jre package as I am trying to learn how to build custom deb packages.