itch.io

No, they won't go away. So you could leave game running and then collect a lot of them in short time because having a lot of Magic would convert other resources faster.

It is an idle game. You idly gain resources, but you can't expect the entire game to be idle. There are always aspects of an idle game that you need to yourself. You can't expect an idle game to be 100% idle in every aspect. If it were, there would be no need to actually play it.

Opening the page, starting the game and becoming the most prolific player ever just by playing once every 24 hours for 5 minutes would make little sense.