Waterproof

Hello ~ iPhones are water resistant and not waterproof. Take a look here:

About splash, water, and dust resistance of iPhone 7 and later

Water and other liquid damage to iPhone or iPod isn't covered ...

~Katana-San~

"Water resistant", not "waterproof". And even that is accomplished with internal "tape" barriers that break down over time.

Could have slowly occurred because of humidity, or a number of small splashes you do not even recall. OR (maybe if it was not original-Apple-sealed) before you bought the phone.

The phone is not waterproof, and Apple never said it was waterproof. It is water and dust resistant, and Apple also says that its water resistance can decrease over time. As the XS was discontinued 2 1/2 years ago your phone is not new, so there is no way of knowing how it was treated in its previous life.

A lot of things could happen, including steam from a bathroom shower or possibly even rain getting in. There are a lot of variables involved including how intact the seals are, since they can break down over time.

There's certainly been criticism of several device manufacturers that advertised a water resistance specification but have expressed warranty terms that water damage isn't covered. And not just Apple. I'll just leave it at that.