Most phones consist of similar chips, so as long as the Linux kernel has support for these, it is possible to run something like postmarketOS on closed devices

Most phones consist of similar chips, so as long as the Linux kernel has support for these, it is possible to run something like postmarketOS on closed devices as well. It is harder, sometimes you need to hack the bootloader to get it done (or kexec-boot from the proprietary kernel like done on the PS4, get creative), but this has been done enough times for various devices. So with enough manpower and dedication at least booting a Linux-based OS is possible (not saying that you will be able to use all the hardware).