How can you have two iPhones sitting right next to each other, one with No Service and the other with 2 bars?
To pre-empt any questions about the iOS versions running on each: the iPhone SE is (13.7) is more up to date than that on the iPhone 7 (13.5.1).
The difference in signal on devices can have many explanations. The devices are different and may have different antennas with different reception. Cases on the device can also affect signal.
You're comparing a phone that retailed at $650 when it launched vs a budget iPhone at $400. Apple cut corners to get that price point and antennas is one way to do that.