Is the iPhone XS Max waterproof?

It's marketed as meeting IP68 water resistance from the factory. However, Apple strongly recommends against testing this and disavows any responsibility for damage as a result of exposure to liquids. They don't seal it like a divers watch. It uses adhesive seals that eventually degrade for several reasons, including exposure to water. Maybe it's back to that resistance after being serviced (like a battery change) but then it starts degrading all over again.

It's more like a case where it might help, but it's not something that anyone should rely on keeping liquid out of an iPhone.

See this, About splash, water, and dust resistance of iPhone 7 and later - Apple Support

No

No.

It is water-resistant (IP 68). Not water proof. Warranty doesn't include water damage. Don't put your phone in water it could be failing.