Background data

Force closing apps will not stop them from using background data. You need to go to Settings>General>Background App Refresh and disable it for the apps you don't want using data in the background.

Closing down your apps, as you call it, is a bad idea and interferes with normal operation of your phone. See→Do not close "background" apps | Communities

It also won’t stop background activity for any apps that receive notifications or automatic updates; those apps just get restarted, but won’t show up on the app list because they weren’t launched by you. The app list in the quick-launch screen is only apps that you have opened, and its purpose is to quickly switch between apps, not to manage apps.

You can follow IdrisSeabright’s advice and turn off Background App Refresh, which will stop apps from launching themselves to do housekeeping, but still will not stop background processing of notifications and automatic app content updates.

9-Rebecca-7 wrote:

My iPhone 11 keeps using background data even though I always close down my apps so I’m not sure why it’s doing this? Seems to drain my battery so can anyone help with this?

Verify you're a up to date:

Update your iDevice - The current stable release of iOS including bug fixes, is iOS 16.0.2

Update your iPhone or iPad - Apple Support