My iPhone is draining even if it’s turned off

How old is the phone? It was discontinued in 2020, so I'd be amazed if the battery still had 100% capacity. Try replacing it and see if that fixes anything

It sounds like you may be experiencing an issue with your iPhone's battery or with its power management system. While it's good news that your battery is showing 100% capacity, it's clear that something is causing your phone to drain overnight from 90% or any other percentage to 0%.

There could be several reasons for this issue. One possibility is that there is a software problem causing your iPhone to consume more power than it should be, even when it's in sleep mode. Another possibility is that there is a hardware issue with the battery or the power management system, which is causing it to drain more quickly than it should.

To diagnose the problem, there are several steps you can take. First, try resetting your iPhone by holding down the power and home buttons until the Apple logo appears. This can sometimes fix minor software issues that could be causing the battery drain.

If that doesn't work, try checking your iPhone's battery usage settings to see if any particular app is consuming more power than it should be. You mentioned that the graph is going down but not showing which app is consuming, but sometimes you can still identify a problematic app by its high battery usage percentage. Try uninstalling any apps that are using a lot of power and see if that helps.

If the problem persists, you may need to have your iPhone's battery or power management system checked by a professional. It's possible that your battery may need to be replaced, or there could be a problem with the phone's internal hardware that is causing the battery drain.