I got excited when I heard the iPhone 13 would support 2 eSIMs. What I didn't realize it's an either or situation. The first SIM slot is either a physical SIM or an eSIM but not both at the same time. I erroneously assumed I could use a physical SIM and also the 2 eSIMs concurrently. Wrong.
You can activate the new iPhone 13's on Sprint with a system-generated eSim.
Or with a T-Mobile physical sim.
But that is the first iPhone model where you can do that - not for older phones that would not take eSims.
It is only allowed where the phone allows IMEI1 to be an eSim.