It is expected to drop a little overnight if you don’t follow the best practice of charging overnight, but if it drops more than 10% or so there is some feature or app that is using excessive energy. You can check for apps by going to Settings/Battery and seeing which app(s) cause the drain. Other possibilities are a very weak cellular signal, having VPN installed (even if it isn’t turned on), or a failing battery. To check for that go to Settings/Battery/Battery Health.
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
