Why is my iPhone X shutting down unexpectedly?

This could very well be the symptoms of a significant degraded battery. Go to Settings > Battery > Battery Health and Charging. What is the battery health percentage? If it is 80% or below, you are overdue for a battery replacement. You should have replaced the battery in your iPhone at least once by now.

• Make an Apple Store Genius Bar reservation: Genius Bar Reservation and Apple Support Options - Apple
• Make an Apple Authorized Service Provider reservation: Find Locations

It's likely a hardware problem.......either the battery or main board.....or in rare cases, both.

Make an appointment at the Apple Store or take the phone to an authorized Apple service shop to let them check and test the phone and give you an estimate of repair cost.

You can find a list of authorized Apple service shops using this link: Find Locations

If estimated costs are high, the money might be better spend on a new phone.