A forever world is just a world you plan to play on forever! Doesn't need to be taken from update to update, though it can if you want to :)
I personally use quite a few mods in my forever world which means I'm stuck on any given update until all my mods are available for the next (Currently I'm on 1.21.4).
I essentially don't make a new world anymore, I just load up this one and shape it almost daily
Yeah sometimes I just browse the mods in the prism launcher and see if there's any that look nice, and I just add them!
I actually started the world vanilla, but can give you my mods list... one sec (and also yeah you could either go to new places or get a chunk deleter but I've never used one of those so not sure how well they work)