KeePassXC is like a total rewrite of the original KeePass password manager.  The developers rewrote everything from scratch so it could work natively on Linux, Windows, and Mac instead of just Windows. But turns out the developers decided not to make their own mobile app because there's already some really good KeePass apps for Android and iPhone. 

For Android, they recommend KeePassDX or KeePass2Android.  Both seem solid based on reviews. 

And on iPhone Strongbox or KeePassium are their top picks.  I checked out Strongbox briefly and it looked slick and simple to use.

Anyway, the key thing is that all these mobile apps are open source and compatible with KeePassXC.  So they can sync up and work together nicely.