iPhone 12 battery draining and not charging

Well your iPhone not charging is certainly abnormal, but your iPhone draining battery, especially when not connected to a power source, is very normal. So this seems to me as a charging issue.

Use a flashlight and either a plastic or wooden toothpick, but nothing metal, to get any lint and debris out of your charging port. Also make sure you are using the charging cable that came with your iPhone or an Apple MFi-certified cable.

You may also want to review this for more information:

If your iPhone or iPod touch won't charge - Apple Support

You may have a poor battery condition which cannot provide sufficient power for peak performance. What is the Battery Health of your iPhone? Open Settings ➡️ Battery ➡️ Battery Health and let us know which percentage you see for Maximum Capacity. Also note what is written under Peak Performance. If your battery's Maximum Capacity shows a value of 80% or less, then it has gone through its lifecycle and therefore should be replaced, according to Apple.

Axel F.