Also on top of what other folks are saying, making a complete modern computer is really fucking hard.
No one is making a 3ghz CPU in their garage. Maybe folks are assembling stm32, or pis, or whatever into their own pcbs, but the machines that make the chips just aren’t hobby level yet. You just have to buy some stuff.
Maybe that’s not your point. Maybe you’re okay with the processor being closed because it is fucking hard to make.
Beyond the hardware now we’re talking making your own drivers and shit. There are resources out there like zephyr, but there’s millions of devices that aren’t covered.
Try writing your own SPI interface for an established MCU and a common periferal. Make sure you include crc, don’t skip it. Maybe skimp and do i2c. It’s fucking hard.