Thank you very much, @Shamino for this considerate reply and links.
I’ll research some more on this but I use my apple devices mostly for email, web browsing, photo taking and handling, RSS reading, .txt note taking, pdf viewing, offline maps and similar. If such apps are out there in Linux open source, fine. Vocabulary and technical knowledge challenges are there, but like anything else, overcoming challenges to reach a worthy goal, is, uh, worthwhile!
Is there a phrase or keyword you can suggest for searching for this kind of computing?