How do I stop apps from running in the background on iPhone 16 Pro?

In addition to what lobsterghost1 has provided, I would go to Settings>Battery, then scroll down and see what apps are using the majority of your battery. That will help you try and diagnose where the problem may lie. It could be one app is having an issue and force closing that particular app will help. A Force Restart of the device may help clear things as well. Force restart iPhone - Apple Support

You may also want to turn off Background App Refresh as well. Go to Settings>General>Background App Refresh. You can turn it completely off, or you can go down the list and turn off specific apps.

You can always force close an app if it continues to run in the background, though apps suspended, should not be running in the background unless there is an issue with the app. Meta and Google apps often have issues with not fully suspending when not being used.

To Force Close an app, with your finger at the bottom of the screen swipe up slowly half way and stop. This will bring up the App Switcher of all apps which you've used. To force close them, swipe the upwards and it will be fully closed.