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Have a broken iphone 8 don’t want to spend money on parts if I don’t have to …I also have an iphone 8 plus with thats locked, could these be swapped to have a functioning iphone

Without some extreme modification, it would not be possible, the board as made for each phone size. Best simply replace the bad part. You may be able to take you bricked iphone 8 plus to a repair store and trade for the replacement piece Take a look iPhone 8 : iPhone 8 Teardown iPhone 8 Plus: iPhone 8 Plus Teardown