Many Android devices, including all the ones that Google make and sell themselves, Sony devices, Motorola devices, have an officially supported path to running any code you want on YOUR own device.

Usually that path is: boot the phone into bootloader mode, plug it into a computer, run `fastboot oem unlock`, accept the warning that this will void your warranty, then `fastboot flash <image file to flash>`. An officially documented and supported way to do exactly what you want with the device that you own.

Yes, it voids your warranty, but it's not forbidden or illegal in any way.