What is the easiest way for multiple people to work on a project?

Me and my friends are all designing a game. However we dont have an efficient way to work on it. Whenever one of us makes any change/update to the game, everyone has to download the folders for it.

We are going to be doing tons of playtesting and making constant changes, and its unrealistic for us to just keep downloading every small change.

Whats the best way for a team to work on a project together? For example, is there a way to save the games files on a cloud storage, instead of on each of our computers, so that any update happens on the cloud, instead of requiring a folder download?

Sounds like you need source control of some kind. Github will host it for free if you don’t mind it being open source, or you can pay money to have it be private.
Alternatively, you could investigate installing and using git locally