Why is the PPP still $400 and no stable OS?

[Kevin Kofler]

this is shocking, i thought oneplus was good at opening bootloaders comparing to other android brands. maybe it is, it is just oneplus is less worse on opening bootloaders.

This is unfortunately a common misunderstanding: "Unlocking the bootloader" does not mean that you can actually replace the bootloader. Almost no Android device allows that (and where it is possible, it is typically due to some exploit, not by design). Not even the Fairphone. The way Google intends the boot "security" chain to work is that there is a public key burned into the SoC (specific to the phone manufacturer ordering the SoC) and that the SoC will refuse to boot anything not signed with the corresponding private key. All the mainstream smartphone SoC manufacturers work that way. All that "unlocking the bootloader" means is that the unmodifiable bootloader will actually allow you to boot unsigned operating systems or chainloaded bootloaders, because by default, not even that is allowed by the "security". (The OS signature check in the bootloader is the next link in the Treacherous ("Trusted") Computing "security" chain.) And in addition to the signature check, there may also be hardware write protection preventing you from attempting to install a different bootloader to begin with.

And as I already mentioned, some SoC manufacturers, e.g., Qualcomm, are even worse than that, because at least some of their SoCs enforce the signatures not only for the bootloader, but also for any firmware that goes through the SoC. And since they enforce the signature of the phone manufacturer, it means that you cannot get the firmware directly from the hardware manufacturer as you normally do. (Not sure whether that is an issue for, e.g., the Fairphone, but it definitely is for the OnePlus devices.)

actually, it is not that surprising. in my last android device, which i'm not going to name. i learned that "oem unlock" is not total user control. it is partial control. fastboot or bootloader still have some limitations.

Indeed. The main limitation is that you cannot replace fastboot, so you are stuck with its bugs and limitations. At best you can work around them by chainloading a better bootloader, which is what the "U-Boot on Qualcomm" effort does.

one key element of pp regular and pp pro is bootloader. bootloader feels like user is in control.

The main bootloader-related feature of the PinePhone series is that you can actually replace the bootloader. E.g., you can install Tow-Boot or a distro-provided U-Boot. The PinePhone Pro also has rk2aw available, which chainloads to a distro-provided U-Boot, and mainly makes it easier to use such a U-Boot on a microSD card. (Tow-Boot requires you to hold a button on boot to boot from microSD.) In principle, you can also install whatever you want, e.g., you can make your own fork of U-Boot. There is no signature requirement.

technically speaking, android is linux.

I think the clearest way to put it is: Android "is" (uses) Linux (the kernel), but not GNU/Linux.