Cellular telephone signal bars are not comparable across devices. They’re an arbitrary and approaching fictional representation of signal. More bars is usually better, but anything past one bar is just “more bars”, and not any sort of cross-comparable measurement. (Same goes for the “analog” gages shown in modern vehicle dashboards, these days.)
There are articles on the ‘net that describe accessing a “Field Test mode” and using that for potentially viewing dBm signal strength measurements, but that’s all dependent on the specific hardware involved, is undocumented, subject to change, etc. Whether any if that is available, or works for the phones here?
TL;DR: if your phone connects and works, it works. If the iPhone does not connect and does not work, then contact your carrier initially and then Apple if it needs a repair, or try a different carrier if the iPhone is working but is not connecting where you need to use it. If y’all want to compare bars, well, have at. Comparing bars is whole lot like comparing pubs. Most folks have their own individual criteria for making that comparison. 🤭