Preface: I know buying a phone/tablet is easier, but I'm doing this project for fun and a bit of learning

Hey guys!

I'm an experienced software engineer and my latest little project I've started on is a DIY smartphone/tablet. The motivation behind this project is to build my own custom toy smartphone/tablet OS and to learn more about how embedded software works.

I feel reasonably confident on the software side, but I feel completely lost on the hardware side and I'm looking for advice.

For starters, I'm wondering what I should use for the motherboard. Should I go with a raspberry pi/orange pi or design a custom PCB? I'm concerned that anything prefab I get will not be compact enough to fit inside the case.

Speaking of which, where do I get a compact case that can fit the motherboard, battery, peripherals, etc.? If I get the components with x dimensions, can I custom fab a case somehow that will fit these components?

Lastly, I'm not sure how to procure a touch display. Do I just buy an phone/iPad/tablet and yoink the touch display? I assume getting the drivers to work in this instance would be a nightmare.

Lastly, assuming I'm completely out of my depth here and this will be extremely difficult to pull off, is it possible to buy a premade tablet/phone which doesn't have any firmware locks and will allow me to completely customize the software?

If anyone has any advice, guidance, or tips I would greatly appreciate it.

Thank you <3