I currently have an iPhone 11 Pro with an ATT eSIM. It sounds from this:
That there is a built in software process to transfer the eSIM between phones and that I will be prompted when I first start my new iPhone 12 Pro. Has anybody done this? Does it work? Any tips or tricks?
About eSIM on iPhone - Apple Support
Learn what an eSIM is, how to set up an eSIM on your iPhone, and how to transfer a physical SIM to an eSIM.
support.apple.com