A lot of people don't now that for years (well at least a decade, actually) Apple policy has been that users can do their own upgrades without voiding the warranty. For hard drives, example. Obviously this doesn't apply for soldered-in SSD drives, I'm talking about spinning drives converted to SSD, or capacity upgrades for those same traditional form factor drives, using third party drives (well all traditional drives are third party for Apple, but you get the point).

Apple has a lot more sense than haters give them credit for. Their policies vary on a case by case basis depending on the specifics of the part and the potential impacts on important factors like security. For example you don't want an adversary or an abusive household member being able to install their own hacked version of the fingerprint sensor hardware on your phone while you are not looking.