How do I get the launcher, and which systems does it run on?

Windows and macOS. There is no Linux build, and the launcher is not a phone app.

Two ways to get it: the Install launcher button that appears in the browser version, or the download on the Multiverse page at creta.world. Both hand you the current build for the operating system you are on, so there is no version number to pick.

Install it only from those places. A Creta installer offered anywhere else is not ours.