How to fix overnight battery drain on iphone xs?

You should be charging overnight, every night; it will extend the useful life of your battery. The absolute best way to get maximum use on a charge, as well as slow the decline of battery capacity long term is to enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight, every night. The battery will fast charge to 80%, then pause. During the nighttime pause the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern. If you enable iCloud Backup (Settings/[your name]/iCloud - iCloud Backup) the phone will back up overnight also, assuring that you can never lose more than the current day’s updates. Here's more information→About Optimized Battery Charging on your iPhone - Apple Support

Yes it's normal. Keep it on the charger overnight.

go to settings>battery>see what is using your battery - close those apps.

going down by 10% in 8 hours is pretty good and should not worry you at all as Kurtosis indicated.

There is nothing to fix. Phones do all sorts of things in the background such as looking for updates, scan for new emails, etc. It is normal behavior.

my wife and I make sure we plug in our phones every night before we go to bed.