Apple warranty

Depends on what the issue is. They might diagnose it and perhaps it's a software issue that can be resolved with the existing hardware. Or it could be diagnosed as a hardware issue as a result of accident or possibly just failure of a component. Worst case scenario is that the issue doesn't get fixed and doesn't cost anything. One exception would be dealing with Apple Authorized Service Providers - some which charge a mandatory diagnostic fee that may be waived if there's an actual warranty repair.

If it's not a hardware issue covered by the warranty, yes.

They will check the sensors to see if it is due to dropping, water ingress etc and if not a user inflicted injury then it should be a free repair .