MainStage does not run on iPad. It is macOS-only.
I could imagine that we might see a MainStage player at some point in the not-too-distant future now that the iPad Air is on M1 and the iPad Pro on M2, but there are no rumours that would point in that direction at this point, and no way to run MainStage at this point.
Camelot is an alternative that some people are using, but AFAIK, it doesn’t come with any sound library and is just a way to run third-party Audio Units, all of which you have to buy. These are a lot cheaper on iPadOS than on macOS, for the most part, but there is nothing else on any platform that approaches the package value of MainStage.