Why did my iPhone 7 (15.5) get stuck in a boot loop, and how can I prevent it from happening again?

Half an hour ago, my iPhone 7 spontaneously shut itself off. The Apple logo appeared with a thin progress line under it and stayed there for several minutes. Then the progress line started to fill, as if downloading something, but when it got to the end, the phone shut down again. This process was repeated twice more. The third time, I got the passcode screen and everything seems to be normal again.

I'm pretty low-tech, so I looked up "boot loop" on the internet and apparently this is a thing and sometimes the boot loop just keeps on going. I wasn't using the phone when it started, and I'm worried that something is wrong with the phone and the next time I won't be able to get my phone or data back. Any advice about how to keep this from happening or what you can do when it does? Thanks!