By default iOS Pages, Numbers and Keynote store created files in iCloud. However you can move the files to the "On My iPad/iPhone" location if you want to.
You can use the Files App to see where things are stored.
Anything in the iCloud Drive location, would be in iCloud, note that if they are downloaded there will be a local copy stored as well, but will not appear in the "On My iPad/iPhone" location. If they are not downloaded, they'll appear there with a small cloud icon on the top right corner of the icon.
Anything in the "On My iPad/iPhone" location is stored locally. Note that many apps will not appear in the "On My iPad/iPhone" location if they do not have any locally stored documents.
Anything in a 3rd party cloud service would be found inside its relevant location if available in the Files App. You can tap the "Edit" label on the top right corner of the Files App in Browse, to manage what apps and cloud services show up there as locations.
The Location screen/browser can be accessed from Pages, Numbers, and Keynote, and any other file that implements the Files App access.
See here for more information: Use the Files app on your iPhone, iPad, and iPod touch - Apple Support