What is a good/highly-rated app to monitor internal temps, battery temps, etc in real-time when the phone is in use, charging etc? It’s not to troubleshoot, I just really am interested in how different things work, materials science/engineering etc.
Unfortunately Apple makes it quite difficult to learn how things work this way. Any app using private APIs won't be allowed on the app store, and normal apps don't have much use for temperature sensors. There are CPU/RAM monitoring apps, but they're not all that useful considering you can't have more than one app open at a time. Profiling via Instruments is the only way to get much of a look at the software side of iOS while it's running.
I can't imagine what it's like growing up today, I only became interested in computers after fumbling around inside early OS X and learning what things do. Nowadays no kid will ever know what a dylib is, because they have no ability to see anything "behind the scenes" on their iPhone/iPad.