“Major carriers still offer basic flip phones.They often have better coverage than “smart” phones,”
SMART phones may have access to more frequencies than basic phones.
Newer flip phones are built to access 4G, LTE, and 5G so it's accessing the same frequencies.
The antenna in a flip phone is installed in the ear half, which allows manufacturers to use an optimally sized one and its location makes it less susceptible to interference from the device itself. Even when the phone is closed, the antenna has full facing versus being buried within a smart phone.
I am not a phone guy but a simple explanation is cell towers and phones use spread spectrum to share the same bandwidth without interfering with one another.