If it takes only 5 seconds, it doesn’t sound like a reboot. Does it show a black screen and the apple logo during this event?

No Apple logo, just black screen with loading spinner followed by requiring passcode to unlock

That might be what's informally called a "respring", where the SpringBoard process is restarted.

SpringBoard is the process that shows the home screen, and does part of the lifecycle management for regular user apps. (i.e. if you tap an icon, it launches the app, if you swipe it away in the app switcher, it closes the app)

It is restarted to make certain changes take effect, like the system language. In the jailbreaking days, it was also restarted to make certain tweaks take effect. Of course, it can also just crash for some reason (which is likely what is happening to you)

Hi, is there some further info on iOS "internals" like this? I was always interested in how it works, but I found much less information compared to android (which obviously makes sense given one is more or less open-source), even though these probably don't fall in the secret category.

mine used to do that when the battery needed replacement

Yes, lots of complaints on forums about this bug. Saw it happen to my phone today.

I always assumed that it was memory reaching capacity or routine cleanup more than a reboot. This often happened to me after intensive use