Can someone help me understand the relationship between phone hardware and carriers in the US? I recently bought a new, unlocked Samsung phone on Amazon. When I put a Verizon SIM card into it, a Verizon-specific firmware module got added. I don't understand why that would be necessary. Asking because I'm attracted to the idea of a phone I truly control, but not sure what that means for carrier compatibility.
I got a Verizon SIM recently and it would not work in my iPhone 8, which I bought in Germany. Fortunately the salesperson thought to ask if it was a foreign phone! Turns out Apple only sells the Verizon-compatible phones in the US, but doesn't exactly broadcast that fact elsewhere.
The Android (or Linux) situation is probably different in the details, but not in principle.