This happened to me yesterday also. I looked at everything as well, restarted all devices & then deleted some new game apps I had downloaded thinking they were the problem, lost my game progress but didn’t repair the problem. Then I realized it was only my game apps that were crashing & closing out, bc while this happened I was in the middle of an Instacart order that was working properly & mail was fine. When I decided to try a game app again after about 30 mins they all opened & worked fine. The game apps were different developers, the only thing in common is they came from the App Store, although so do all my apps & those that weren’t games were working.
I was google searching this morning to try & find answers & came across this thread. I would like an answer to this mystery & to never repeat it.