Ask HN: How to manufacture a programmable mobile device?

Very basic question:

How would you source hardware that could be used as a functional mobile phone, where you build your own OS / applications on it?

The research I've done so far has mostly turned up Android Open Source Project derivatives. A couple examples:

* GrapheneOS, which is security focused

* Parental-control oriented phones like Gabb

What if you did not want to use AOSP? If you wanted to build a mobile OS from scratch, what is the path?

To start, how would you even purchase the hardware? Are there manufacturers that will do this? Who are they?

My understanding is that it would be easier to start with a certified baseband system. That's OK with me. I'm trying to get at how a person could actually go about building their own working phone in real life.