What takes place during an iPhone hard reset?

In response to an iPhone 7 freezing issue I'm having, it was suggested to try a hard reset (holding power + volume down for 10 seconds for iPhone 7 or holding power + home on older iPhones).

This got me wondering what an hard reset actually does. After searching around online, I've sound some that say it is just a hard power-off (isn't this what holding just the power button does?) and others mentioning something about a library being rebuilt. Overall, many, at least from several years ago, cited it as a cure-all for various phone issues.

Does anyone know what a hard reset does? I'm envisioning something analogous booting a Mac in safe mode or doing a PRAM/SMC reset on a Mac, but online searches so far are leaving me unsure.

I do not know for sure that this happens but I was told that in order for IOS to do anything except just it's usual booting functions you actually need to do a hard reset three times in a row.

Not sure why, and this was advice I read (don't remember where) several years ago. Perhaps 3 hard resets in a row trigger some sort of flag that forces IOS to do some special maintenance, not really sure but I am throwing it out in case someone else has heard this or knows is it complete and utter BS.

I actually wrote this up for the tag wiki today, so here you go.

The hard reset process on iOS is akin to a power cycle on a computer. In that process, RAM is cleared, pending operations are completed, etc. For iOS, data marked for deletion is permanently deleted during a hard reset.

This includes app data: If you delete an app and reinstall it without doing a hard reset in between, all data is immediately available again as if the app was never removed. In cases where something within the app's data or preferences is corrupted and causing problems, not doing a hard reset will cause the problems to continue, since the same data will be in use.

An iOS power cycle is closer to the hibernate function on a computer, where all information is written to non-volatile storage and restored when the computer is taken out of hibernate. On iOS devices, the only real benefit to a power cycle is getting fresh network connections.