The way most games work here is that you you pay once and that gets you lifetime access to that game and any future updates.
Technically you buy a project and you get lifetime access to all files that are on that project.
Most sellers consider one page = one game and you get access to all future updates of the game. However it is up to the creator to decide and if someone wants to sell updates as a separate project that is possible.
There's also some danger involved as some less scrupulous people remove all files from a page, even if you paid for it!
To summarize: You buy access to a "project", a project contains a set of files that the owner can change at any time, adding more versions or even removing all files.
