There’s also KeePass2Android. I opted for this because it brings a very useful feature called QuickUnlock. Your opened database gets locked in standby but you can reopen it with just the last 3 characters without needing to retype the whole passphrase.
That was very likely Street Complete which is self explanatory and a great way to start. You are adding all kinds of useful information about any kind of object or building with this.
There’s also Vespucci which allows to alter the whole map (ie edit streets, POIs and so on) but takes a lot more to get familiar with.