Hi Paul
Welcome
1) Arturia software is Big Sur compatible, but not M1 (Apple Silicon) compatible. So it depends what MBP you've got.
2) Arturia Audio Units will run in GarageBand (personal experience). GarageBand may warn that you need to lower security levels to use them, afterwards it'll work fine.
3) As long as your MBP recognises your Korg as a midi controller, AL5 can use it. Does GarageBand already work with your Korg? If so, then that's a good sign AL5 will work with it.
4) AL5 comes with over 2000 presets chosen from Arturia V Collection + Pigments. There is also an additional online store accessible from the AL5 app where you can download hundreds more presets, many of them free. It will be a quite some time before you run out of ideas.
5) Download the trial and see if it meets your expectations. Arturia generously allow users to download a fully-operational unlimited trial so you can test it for yourself before you commit to purchase. The only thing you can't do with the trial is save your own presets, and after 20 minutes it goes mute (so quit the app and open it again).