boot loop – What are the physical causes of motherboard-related bootloops?

A lot of Android phones, from the Nexus 5X to the LG G4, have suffered from hardware-related bootloop issues.

I have recently purchased a Google Pixel XL first gen, and want to minimize the chances of such issues, and so I’m wondering what the possible physical causes of hardware-related bootloops / hardware failure are?

For example, it’s been rumored that the 5X’s problem was the cracking of solder joints from repeated heating and cooling.

I will be using this specific Google Pixel XL as a backup server, and so won’t undergo regular usage patterns, as it can essentially stay on 24/7 with the screen off, or only be turned on occasionally. So what would the best conditions be for such a usecase in order to minimize the risk of any hardware issues?

Should I leave it on 24/7? It won’t get hotter than 30 degrees or so, and cools down to around ambient temperature (20) for most of the time when not in use. Or is it better to only turn it on every couple weeks?