You can repair your phone anywhere you want including doing it yourself. And you can void your car warranty by making modifcations.
Modifications do not automatically void a car warranty
under the
Magnuson-Moss Warranty Act, a manufacturer must prove a modification directly caused the failure of a specific part to deny a claim. While some modifications like cosmetic changes or basic bolt-on parts (e.g., wheels, suspension) are less likely to cause issues, performance modifications that alter engine, transmission, or drivetrain components (like engine tuning or turbo kits) can lead to a denial of warranty for those affected systems. The manufacturer can only deny coverage for the parts that were directly impacted by the modification, not the entire warranty.