One feature of iOS and iPadOS 13 was built-in support for fonts (Settings > General > Fonts). But as Michael Potuck notes, I’ve seen very few font apps in the App Store. But now there’s a new one called Fontcase, and it’s open source, too.
Installing custom fonts is super easy with Fontcase, once you have what you want in iCloud Drive or Dropbox, you just import the fonts in Fontcase, download and install a configuration profile, and they’ll be available across iOS/iPadOS.
Check It Out: Finally, an Open Source Font App for iPhone and iPad