You have to open it at least every few days. Only Apple apps can work reliably on iOS.

That is an excellent example of Apple's anti-trust behavior.

I’ve never once noticed this. I go weeks without opening Google Photos app, but my photos are always there.

Set and forget. All my phone photos are “just there” in Google Photos app and on web.

You can just let it run.

Unless things have changed, yes but no. You have to leave the app running, and turn off display sleep/lock so the phone is always awake. Which practically means it has to be plugged in. It's a major pain. As someone else commented, a classic example of Apple limiting background sync in the name of "stability and battery life". That has a grain of truth to it, but let users make that choice!