Keep in mind that there are two types of phones that were made because the carriers require different antennas on the devices to work with their service.
While true for newer and much older models, iPhones from the 4s and up until the 7, had both types of Antennae built in to all models so can work interchangeably on both types of carriers. It's just that most CDMA carriers will most often not activate a phone on their network that is not in their database of devices, and iPhones not intended for them are in most cases not added to the database during the manufacturing process.
i tried putting in my activated verizon sim card and it didn’t have a message pop up saying that it doesn’t work with the phone. it showed the service bars but not “Verizon”
That generally means the phone is unlocked, however, Verizon would need to add it to their CDMA service for it to work entirely.
If the "Sim Not Supported" error was not shown its unlocked. It won't simply show no service, if the phone is locked and the sim card is not from the carrier it is locked to. It will issue the error in all cases.
You can try a sim card from a GSM carrier such as T-Mobile to verify this.