From what I have heard, Apple could have used Samsung's natively-developed LTPO technology for the iPhone 12, but they felt it was not up to their standards. So as someone noted up-thread, Apple is probably using their own LTPO technology for the iPhone 13 and having Samsung Electronics manufacture it for them as they have the production scale Apple needs (where LG does not, even though they have been using Apple's LPTO process on the Apple Watch).
We saw something similar with the iPhone X. Apple used Samsung Electronics to create the OLED panels which utilized technologies developed by Apple and exclusive to the panels made for Apple and those Apple panels were higher quality than those Samsung Electronics was supplying to Samsung Mobile for the Galaxy series (as DisplayMate testing subsequently showed).