>it's the first time I buy something on itch.io
Yeah, so you asking about the site which is... Not very strict.
Generally if you buy something on itch you get all versions that are available here, if they are all categorized as the same game, you could upload Android and PC as seperate games if you want people to choose (4min only made full version android and demos available here, for full version PC you have to go to steam or subscribestar)
Generally if you buy on itch you get the updates as well, but there's many ways to change that if the Dev doesn't want that to be the case, some even upload every update as a new game exactly for that reason (I doubt 4min would do that or even anything similar but that remains to be seen)