As @Jesse Hooton said.
You need a heating platform. You can do it with hot air, but then you take huge risk of overflowing underfilled IC. Please keep in mind that after that you will need to use a stencil and reball the 2 motherboards back together which is going to be very hard to do with hot air without reflowning undefilled components once again.
The separation and reinstallation is very advance micro soldering and should not be attempted unless you have much much experience.I'm not trying dismiss your abilities. I don't know what they are. I just want to make sure you don't bite into more than you can chew.
Get a heating platform. Mechanics sells them for about 70$ ish. They work well. Just make you have done some before on tester devices if this iPhone 12 is meant to be repaired because it is not easy to reinstall. (This coming from someone that micro solder for a living)
But to answer your question, if you were to use hot air (which I reccomend you don’t) you would use about the same setting that you would use to remove the shields commonly found in iPhone 5/6/7/8 covering mostnof the motherboard components. The solder holding the 2 motheboards have a lower melting point then the rest lf the components just like those shields.
I hope this helps. Please get a hot plate ;)
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