When you buy the game you'll gain access to the latest version through your profile, not on this page.
Also it does not mean you'll get the latest version just because you buy on here, its up the dev.. The dev that creates "Business of Lovin'" updates the Itch.io version of his game waaay beyond everywhere else. Kind of annoyed by that.. I paid for the game, update it everywhere.. Not just on your Patreon..
Anyway, I digress. By paying you'll get the latest version of the game that is uploaded onto Itch.io
Yes once you buy it you'll get access to all the future versions, even if I increase the price of the game. You buy it only one time if you prefer UwU
Also the weird download library is probably because of the bundle system. I have to do the same with Bundles I bought. It's just how itch.io works I guess but once you're used to it it's pretty simple :3
About the updates, you have to download the new versions of the game yourself. You keep your saves anyway and can delete the previous version.