BYOD (bring your own device) and BYOP (bring your own phone) mean you bring your unlocked, compatible device to AT&T. Although you didn’t buy your device from AT&T, you can activate your unlocked phone on an AT&T plan.
SIM cards are small plastic cards that are inserted into your phone. They have a unique ID number and store your personal data. An electronic SIM is called an eSIM, and it can be activated over the internet. All wireless phones have SIM cards, but only dual SIM devices have eSIMs.