As example I don't know is Debian Buster and Focal Fossa (20.04) compatible? How to check it? What are another moments of cross compatibility?
What should to know a newcomer who wants to use Debian repositories in Ubuntu?
As example I don't know is Debian Buster and Focal Fossa (20.04) compatible? How to check it? What are another moments of cross compatibility?
What should to know a newcomer who wants to use Debian repositories in Ubuntu?
Simple answer: You shouldn't.
Don't mix repos from different distributions - they might have different dependencies.
Instead, either uses PPAs, or manually install Debian packages (that you have carefully curated and ensured dependency compatibility).
Anything else will probably not end in a good way.
If you really need to use Debian repos, you should use Debian.
As an example, I've manually installed and used Nano from the Debian sid (unstable) repo for the last 2 years without any issues at all. The dependencies check out, and this ensures I have the latest Nano version available as a .deb
package, instead of being stuck with the Ubuntu LTS version. Also, this is fairly harmless and easy to rollback if anything unexpected should happen.
No; Ubuntu 20.04 LTS was the 2020-April release and is made up of packages from Debian sid, Debian testing, and possibly some Debian stable, but also packages from further upstream.
Debian old stable (Buster; Bullseye is current stable) is from a different year with a different software stack. Some packages may align perfectly; but the majority will not - thus result is dependency hell unless you're very careful, pin & do your homework pre-install, and monitor it from then on (ie. a maintenance burden)
If you're asking the question, you're not ready to fix the problems that are likely to occur, if not at first, but sometime into the future (ie. aforementioned maintenance burden; ensuring changes haven't occurred or fixing the mess after they've happened)