Must both cable and charger be MFi certified to protect iPhone warranty

Apple does not certify USB-C cables or chargers, as USB-C is a universal standard, not a proprietary Apple protocol like lightning was which they did certify, as such you won't find any MFI USB-C cables or USB-C charges for an iPhone 17.

As long as they adhere to USB-C standards they will be fine and should not void warranty.

jpgriffith.gtc wrote:

If my cable is MFi certified, does the charger have to be MFi certified also to protect my iphone warranty

No, but if you buy a garbage-grade power supply and it fries things, you might also want AppleCare coverage.

The Apple warranty is for manufacturing defects, not incidental damage.

Most incidental damage is covered by AppleCare, variously with a deductible.