It used to be easy to do yourself but it's becoming increasingly difficult and risky given all of the advanced parts that can be damaged or improperly put back when closing it.

And starting with the 12, Apple now requires authorized repair shops to use heat to open the phone, something that was previously only done for iPads and was prone to causing repair problems for the casual tinkerer.

https://www.macrumors.com/2020/10/22/iphone-12-heated-displa...