It runs its own native Sailfish apps (there's few hundred of them) and Android apps (you can install Google Play, if you follow a bit cumbersome instructions). However, the UI logic in native apps is different than in Android apps. Native apps use a lot of wiping gestures, while Android apps use more tapping and the back button.
There is also a native web browser (built on top of Gecko) and it has its own UI logic (the usual web browser logic with a back button, no gestures).
So, all together, it has three way to navigate, which makes the whole experience a bit fragmented.
But, if you want to escape the surveillance riddled Android and iPhone, Jolla is a good basic phone. It offers a lot for hackers, because its built on top of Linux and you have access to almost everything without rooting. I got my phone three years ago and I'm still getting OS updates regularly, which is also nice. I just wish the UI was better.