Depends on if you’re using an esim or physical SIM. The IMEI is the serial number of the cellular modem, so one cell phone service per IMEI except eSIMs can theoretically support multiple eSIM profiles, so each profile gets it’s own IMEI to prevent conflicts. eSIM modems instead have a different unique hardware identifier. Ultimately it gets…complicated fast with eSIMs and some of it seems to just be down to how it’s implemented in the real world rather than theory

But in short, IMEI is a hardware serial number to identify the cellular modem, and all of the phone manufacturers have collaborated on non-conflicting IMEI numbers so you can identify the make, model, region and sku of a device based entirely off of its IMEI