"an OS that behaves like a real computer that you can put on an old/cheap commodity phone": https://postmarketos.org/

"an app that creates a programmable environment layered over and isolated from the suffocating mobile OS": Android Virtualization Framework (AVF) on newer Android versions provides a hypervisor and a hardware-accelerated graphics (VirGL) for AVF virtual machines, allowing users to run an isolated Linux GUI desktop with low overhead.