You can use your phone while it's charging, nothing wrong with that.
You can't 'overcharge' your phone.. there is nothing wrong with leaving it plugged in overnight, in fact I'd recommend it along with the 'Optimize Battery Charging' selection.
IMO, the worst thing you can do to your iPhone's battery is to let it down below ~15-20% on a regular basis.
Have you looked at Settings > Battery to see if it shows a certain item taking up a bunch of energy?
If it shows something that you aren't currently using, you could kill the app
Close an app on your iPhone or iPod touch - Apple Support
Pretty new phone, but might as well check the battery health? Should probably be 99-100%.
iPhone Battery and Performance - Apple Support
About the battery and performance of iPhone 11 and later - Apple Support
Do you have poor cellular reception? My battery drains worst when I'm at work in a clean room where there is no cell reception. If I remember, I put my phone on airplane mode when I'm in there so it's not constantly trying to connect.
Here are a couple of other articles about batteries you could review
iPhone Battery and Performance - Apple Support
When to charge your iPhone or iPad - Apple Community