Data Saving Apps & General Ideas

What general ideas and alternative apps are good to keep in mind to reduce usage of mobile data?

Thanks to @Chelle many of us use Facebook Lite instead of the bloated regular Facebook app. And instead of browsing the web with Chrome or Firefox we use Opera Mini and Puffin. Between Opera Mini and Puffin, which is better at using less data?

What other apps are worthwhile using towards achieving the goal of minimizing data usage? What other user behaviors can reduce data usage? If you use Chrome or Firefox when on WiFi, does leaving it open in the background when on mobile data even without using it, leak data usage? Do other apps leak data while open in the background?

Opera Max, another @Chelle recommended app, is certainly very helpful in data reduction goals, but I've found sometimes it freezes up my data connection and I cannot use the internet until I restart Opera Max. Anyone else experience such behavior? Is using Opera Max to block certain apps from accessing data more effective than blocking said apps directly in the Android settings?

I've noticed some apps that I rarely use, but need installed for the few occasions it's needed, leak small amounts of mobile data consistently. Is there any way to granularly control them?

Android let's you disable background data either by app or totally.

I'm new to smartphones in general?
Iv'e got a samsung rev something...
I can find a screen with a graph for notifications and disabling data , but can't seem to make it work.
As for finding how to turn on and off background apps. no joy.
any info appreciated.

True data misers will also turn off background data and/or stop auto-syncing.

I think Puffin uses less data but Opera Mini is better at playing YouTube videos in low bandwidth situations. (Although I haven't been successful at getting either to work acceptably at 64 kbps. Puffin won't play YouTube videos at all and Opera Mini has a LOT of buffering.)

Turn off background data for individual apps or for all apps? And what auto-syncing is it worthwhile considering disabling?

Depending on what you have configured to sync, your phone might be syncing emails, text messages, photos, appointments, fitness and other app data, bookmarks, contacts, documents and other cloud drive files.