Is the iPhone XR waterproof?

No iPhone is waterproof. Most models are water-resistant to various degrees. That doesn't mean you can put them underwater and expect them to come out as if nothing happened.

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

According to Apple:

This model is splash, water, and dust resistant and were tested under controlled laboratory conditions.

Also important:

Liquid damage is not covered under warranty, but you might have rights under consumer law.

Definitely not. While Apple markets it as water resistant, there is no coverage for water damage.