The difference depends on if you are talking about what is legal (eg right to repair) or what is possible by the consumer without modification. Legally you can do whatever you want since you own the hardware - but in both Apple’s case and Lexus’ case, installing a non-OEM part will void the warranty on that part, or in Apple’s case, the entire kernel since you’re modifying that kernel (although it’s easy to factory reset an iOS device and remove the modification without a trace).