Where do I start in making my own functioning iPhone?

Everything is possible, it depends on how much time and resources you’re ready to spend on the project and quite much what’s the purpose behind it, as if it’s just for the fun of it the thing might be lots of fun, but if it’s just money saving what you’re looking for you can forget about it. In any case you’ll need a working and unlocked logic board+home button. If you mean making a phone at even lower level, you’ll need at least a working and clean Soc, Nand memory, Eeprom, Baseband and home button. Most other parts can be picked from other phones you may find for sale for spares and repairs.