Core updates

One day I updated core20 but now I'm asked about core 16 and core 18, should I update them or not? I feel like I'll downgrade. Then I ask it because info about changes never loads so I couldn't read them to have more details and maybe choose what to do by myself.

Those look like Snap (Core) and possibly flatpak (Core 18) dependencies.
Installing will not downgrade anything, as they gather all the dependencies.
You would end up with Core 18 and core 20 both installed and called upon as needed by the flatpak app.

It is easy to see how that can be confusing since APT packages rely on the dependency, not having each version of the dependency. So if an APT package depends on libexmp-4.1 and you have 5.3 installed, it still works without trying to install both libexmp-4.1 and libexmp-5.3.