I would love it if I could build & maintain my own mobile computer (AKA smartphone) just as I would build my own PC. I would love to build and maintain my own

I would love it if I could build & maintain my own mobile computer (AKA smartphone) just as I would build my own PC. I would love to build and maintain my own PC, but I assume that Intel Management Engine (ME) or the equivilent is going to come with the CPU for the PC I build.