Your first port of call should be the Battery Health page (System > Battery > Battery Health). Here Maximum Capacity should be above 80 percent, and you should have Optimized Battery Charging enabled. If Maximum Capacity is less than 80 percent, or you have a message under Peak Performance Capability about unexpected shutdowns, then there's a problem with the battery and you need it looked at.
Next, take a look at what's shown on the Battery screen (System > Battery). The charts here is very useful, and shows a lot of information. First, we'll look at the Battery Level chart. Look at the slope of the discharge chart. The steeper this line, the faster the discharge.
It will also show you when your iPhone is charging up (green lightning symbol and hatched green lines).
It will also show if Optimized Battery Charging is working because you'll see the charge level go up to 80 percent and then flatline until the morning.
Times when your iPhone is in Low Power Mode will be shown in yellow.
Underneath Battery Level is Activity. This chart shows you what's consuming the battery, both apps running on-screen (shown ion dark blue), and in the background (light blue).
Below that you also see which apps are running.
You can tap on the bars to get the activity for that period.
Here I'm noticing a couple of apps that use Bluetooth -- Tile and datAshur BT -- doing a lot of background activity while on charge.
This high background usage continues for several hours.
But the activity tails off once the iPhone is off charge, so this is not going to be draining the battery, and may be a feature of Bluetooth apps. Or might be a Bluetooth bug in iOS 14.
If this was happening when the iPhone was no on charge, this could be a serious battery drain issue.
Unless you have an app specifically designed to run in the background (such as a music player when the screen is off) then background usage should normally be low.
You can also get a breakdown of usage over the past 10 days by tapping on Last 10 Days.
You can also switch between battery usage as a percentage and time by tapping Activity/Show battery Usage. This tells you how long apps were running, along with a breakdown of on-screen and background time.
If you find an app that's running wild in the background you can either delete it or, better still, prevent it from running in the background by going to Settings > General > Background App Refresh and temporarily turning it off by flipping the switch on it. Note that this might cause the app to not work properly.