What should I do if my iPhone 8 battery keeps draining?

My iphone 8 battery is draining too much. One day I charged it 100% before going to sleep and I switched off my device. The morning I found my iphone completely dead. it repeats 2,3 nights and I search for the solution. A influencer told that try to factory reset the device. When I factory reset it, the problems where solved for temporary I got the peak battery backup that day, and then the next day I found the problems again. I tried to find the solution in a third party service centre, the used it for 1 day, then said there is no problems with it. And returned. After some days I got the problems back.

I tried all the settings,

But there is no changes.

Is there any tips please?

[Re-Titled by Moderator]