Just to add to @0591ryan, the Librem 5 product page states:
Purism’s Librem 5 phone will be able to run HTML5 applications, which means common applications you use everyday will be available through the web browser. Purism will be able to isolate these applications from the OS, so the applications will not be able to have visibility into other running applications or have access to areas that you do not allow.
So those apps that also have web site versions should be fine. Anything that exists solely as an Android/iOS app won’t work. That said, there is a project called anbox that seeks to allow running Android apps from within GNU/Linux. The functionality probably won’t be there at launch, but it’s at least a project people are working on.