[mras0]

Simplified: Traditional SIM card is "smart" card with a chip (exactly like modern credit cards) from your network operator that gives access to mobile network(s). The chip has software + data that allows this.

eSIM: same (or similar) chip is soldered into your phone. Software is different, and allows storing the data part from multiple operators at the same time. You use your phone to switch between operators and add/remove potential operators (add part is over the internet via your phone, either via wifi or current active operator).

By way of analogy you can download new "SIM cards" to your esIM, switch between them, delete them etc. It's a feature of your phone, either it supports eSIM (has such a chip and supporting software on the phone) or it doesn't. You can't add it later.