OSM is not really a map app, it's a collaborative geo database. You can build apps of top of it, there are many many such apps. The https://openstreetmap.org/ website isn't really a map app and is mainly tailored for the needs of OSM contributors and editors.

CoMaps is a map app that uses OSM data, and packages it in a nice, offline-first, private, and Open Source app that is quite good and easy to use.

https://cartes.app/ is another app that uses OSM data, but works closer to your regular maps, being online-first, which has its pros and cons. Still quite private and Open Source though.