Nexus 6P

Bootloops: This is a common problem, often referred to as "BLOD" (bootloop of death). It is caused by four of the eight cores stopping working. There is a workaround, but you need an unlocked bootloader—if your bootloader is locked and OEM Unlocking is disabled, do your best to get into Android for long enough to enable OEM Unlocking, then unlock the bootloader with Fastboot. Once the bootloader is unlocked, this guide on the XDA forums can help.

Any other problems: There is probably information on the XDA forums.