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
An eSIM is an industry-standard digital SIM that allows you to activate a cellular plan from your carrier without having to use a physical SIM.
support.apple.com