crosscompiling chain?

[ryo]

To answer it as simple as possible: there's no such a thing as a cross compiler for a Pinephone, you compile directly on the phone itself.

Install and enable SSH, download TAR packages, ./configure, make, sudo make install.
Or if you develop Linux apps, simply use whatever you've been using on PC Linux in the exact same way (gcc for ANSI C, g++ for C++ etc).

In case it sounds confusing, maybe this statement clears it up:
Android and iOS are consumer toys that require a PC in order develop and run your own apps on, and there's a huge walled garden for all other apps.
Pinephone and Librem 5 are technically full laptops in a phone form factor, so app development for those should be treated like app development for desktop and/or server.