Ask HN: How do I build a Linux phone?

I have zero knowledge about what it takes to build a linux phone, please help me understand what I need to learn to be able to build one.

I can spare maybe $20k to experiment over the course of my learning and can travel to shenzhen if required.

How do I go about doing this?

I want to start by installing ubports and postmarketos on a device like the nexus 5 and understand how the OS works. What else would I need to learn to reach my goal of building a linux phone? A learning path would be greatly appreciated. Thank you.

Somewhat related : is it possible to stick a raspberry pi compute module in a phone form factor and boot raspian? That would be a cheap way to start.