Could I put a iPhone 11 motherboard in a iPhone 12 frame?

The answer to this is the same as it is for any other proposed motherboard swap on an Apple phone; i.e., no, you can't.

The thing is, For each and every model of phone that Apple makes, the motherboard is custom designed for that phone and that phone alone. There are no cases where they've said, hey, let's make this motherboard compatible with the previous generation.

Not only is the motherboard made in a very specific shape to fit in a certain space, the connectors are placed such that even if you can physically fit the board into the case, the connectors either won't reach where they need to go, or else they will have changed and won't even plug in. Even worse, there may be cases where Apple has used the same connector but changed the pinout of the connector due to changes in whatever it's plugged in to such as cameras, microphones, etc.

So the short answer is no, it won't work.

@yanaolesh If you're asking whether you can swap, say, an iPhone 12 motherboard into another iPhone 12 frame, then the answer is yes and no.

Yes, physically you can swap a motherboard from one to another as long as it's the same model. Meaning yes, you can swap an iPhone 12 to another 12, but that doesn't apply to other 12 models like the Pro, mini or Pro Max. Those are all interchangeable only within their own model, and not to any other.

However, that comes with a lot of restrictions. Apple is continuing their policy of pairing parts to the motherboard such that only Apple can replace these parts and maintain their full functionality. For example, if you swap only the motherboard, you'll lose Face ID, Battery Health, True Tone and auto brightness. In order to keep those functions, you would also have to move over a significant percentage of the components inside the phone, including cameras, batteries, screens, and various sensors.

So yeah, it's getting to the point that swapping out a motherboard is less and less of a viable option for solving logic board issues, including iCloud locks if that's what you're thinking.