In my experience the most common cause of a bootloop an an iPhone 7 is the Audio IC chip… or rather the lack thereof. This chip will loose some of its solder joints similar to how the touch IC would on the iPhone 6 plus.
Here are some images showing where it is and how to get to it.
[image|1643366]
[image|1643369]
[image|1643370]
The only real way to fix it is to remove this chip, re-ball, and re-solder it back. This does come with some risk and should only be done by an experienced professional.
Also, with some of the later updates of iOS, most iPhones will boot up showing only a black or white screen before showing an Apple logo. This alone doesn’t indicate any kind of issue with your phone.