WhatsApp Draining a Huge Amount of Battery in Background

WhatsApp has had some very bad coding on the apps of late. WhatsApp is not owned by Apple (Meta owns them), so if it is a root cause of battery usage, contact them to let them know. In the meantime, I'd recommend when you use WhatsApp, after you finish what you are doing, force close the app, so it can't continue using power. To force close an app, take your finger to the bottom of your iPhone screen and keeping your finger this slow slide up about half way up your screen. This will bring up the app switcher. Release your finger, look for WhatsApp and then slide the app up to fully close it.

Name one application that Meta owns that does not trash your battery?

You will find the same thing with Facebook and Instagram. Their whole app development process is plagued with bugs. WhatsApp has been updated 9 times in the last month and they can't seem to be able to release an update that does not cause another problem.

Lobstershost1 is correct, to completely shut the app off on your phone, you need to Close the App.

Close an app on your iPhone or iPod touch - Apple Support

The ultimate solution will either come from Meta to fix the app, or from you to Delete the app.