Is it possible to make an INF money script?

Very unlikely to make an "infinite money script" as you are trying to set that value from a client sided script. The money is likely done on the server, therefor that value isn't affected by a client sided script. If your game your trying to create this script for is well maintained it's likely impossible, certain games use remotes to interact with the server for money.

If your game uses remotes it's very well possible (spamming the remote) to check this you can use a remote spy (Hydroxide etc) and confirm if it uses a remote. This is again unlikely as I've only seen it twice. And both games were very poorly made and had very low player count.  

And to answer your other question, this can be done on anything, mobile or windows. Whatever you can use or works the best for you.  

Have a good day mate.