Among Us it free on places like Steam, the App Store, and other places like that. You can get it free in places like there if you want.
Oh, yeah, I remember ! They recently removed ads for a time because some people were getting unappropriate ads. They will enable them again some day, but until then, there are no ads on mobile.
Although, some people say they never got ads since the game released in 2018. There are reports that the ads sometimes never trigger.