Battery Drain Overnight

You need to be using the Optimized Charging option in the Battery settings and charge every night. This setting is designed to learn your charging patterns and will only charge your battery to 80%, then slowly charge to the full 100% shortly before you normally take it off charging. If you do not follow a repeatable pattern for charging, it can never learn your behavior and will resort to quick charging your battery to the full 100% every time, which generates more heat and damages your battery more than relying on the system recommended behavior of Optimized Charging.

As for the 6% drop overnight without any use, this is totally normal. The phone still consumes energy from the WiFi, Cellular, and Bluetooth antennas. In addition it will check for updates and download them too, even though it would wait until it is plugged in to install the update. Your phone stays busy whether you are actively using it or not, even when it is shut off it is still trackable using Find My.

So Charging My Phone Upto 100% Overnight Won’t Affect My Battery Health. Is That What You Are Saying?

This will help prolong the life of your battery and is recommended by Apple. Please read what battery expert Lawrence Finch has to say:

There is never a time when any smartphone is doing nothing, so it will use energy 24/7, and the amount used will vary from day to day and minute to minute. For that reason alone you should charge your phone overnight, every night.

In addition, 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. 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

So Charging My Phone Upto 100% Overnight Won’t Affect My Battery Health. Is That What You Are Saying?

This will help prolong the life of your battery and is recommended by Apple. Please read what battery expert Lawrence Finch has to say:

There is never a time when any smartphone is doing nothing, so it will use energy 24/7, and the amount used will vary from day to day and minute to minute. For that reason alone you should charge your phone overnight, every night.

In addition, 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. 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

Keep the phone connected to the power adapter all night......every night.

That is the single best thing that you can do to preserve the battery life on your phone.

Sometimes, the things that you have heard from the "experts" on the Internet are wrong.

It is not possible for the iPhone to overcharge......because the charger is in the phone. We can provide more proof for you if you are interested in the the longest life for your battery.