Thanks for explanation, this was very helpful. One more question please. I can do this restore via Finder with any IPSW anytime if Apple still signs version of iOS I want to install right? For, example today I can install iOS 15.1 RC with specific IPSW file and tomorrow I can download iOS 15.0.2 IPSW and restore my iPhone, right? No need to go to recovery mode etc?
yep. I’ve upgraded and downgraded my iOS using the “check for update” button holding alt select the IPSW.
That’s quick and easy, then if that doesn’t fix your issue, do a back up to Mac, then hold alt click restore and select the IPSW so your able to restore back onto the iOS your on and not letting finder do it by default to the latest official release.
difference is first option just wipes the iOS only and installs one of your choice, keep it to the same version though, 15 to 14 you might get trouble, 15.? to 15.? has been fine for me, And your apps and data aren’t touched.
2nd option, restore, wipes your whole phone and reinstalls the iOS via IPSW and then installs your latest back up so reinstalling apps and data. That can sort a whole host of issues out.