Found this (old) article that gives some information on the subject.....
https://www.cnet.com/how-to/11-ways-to-trick-android-into-using-less-data/From article....
"Restrict background data
Even apps that don't allow you to fine-tune data settings could still be loading background data. In Ice Cream Sandwich and later versions of Android, one way to find out which ones are guilty is to go to Settings > Data Usage, and scroll down to reveal a list of apps with accompanying data usage stats.
Then, tap an app to view its usage data, and take a look at the two numbers next to the pie chart. "Foreground" refers to the data used when you're actively using the app, while "Background" reflects the data used when the app is running in the background.
If you notice an app is using too much background data, scroll down to the bottom and check "Restrict background data." Just note that this setting overrides any conflicting app behavior (like an app that would otherwise update your bank account info every few hours)."
Hopefull turning off the background data switch is the key.