what do you use to internally clean an ipad?

If you are referring to the Safari browser cache, tap Settings > Safari then tap to clear: History and Website Data

There is no "cache" per se for the iPad itself.

Apps have their own cache within the app itself if the app developer sees a need for it. iOS does it's own housekeeping and will clear the cache as it sees fit. There are a few apps in the App Store that can clear this cache but they are becoming few and far between (there used to be tons of them). The reason being is that Apple is slowly (I believe) removing these as it finds them because app developers can inadvertently and incorrectly use their app's cache folder to store (sometimes critical) user data. If one of these cache clearing apps is run, it has no way to tell exactly what kind of data is in the app's cache folder and simply wipes out the cache...and the user's data contained within. Not a good practice as far as the app developers go.