Which Java GUI shall I learn in 2021? I see people are demonizing swing and telling its obsolete, so it would be really helpfull if someone gives an nice Java stack example for developing windows application focused on PLC and other external controller like arduino, raspberry etc, control and visualisation., in general, automation and industrial monitoring and controls.

Just use c# instead

Recommended GUI library for java is JavaFX.
That said, i don't think you can run java (or c# for that matter) programs on arduino, and you may need to tweak jvm settings for earlier versions of raspberry.

You shouldn't.

Java in 2021 (and for the last few years) is all about the backend. Frontend is now almost all just the standard web stack.

Jobs exist which use frontend Java GUI libraries of course - but they're almost all legacy crap, the type of stuff you want to stay away from. If you want to build up your frontend skillset, you'd be better off learning web stuff instead.