You would be surprised with phones.
All depends on the app, but even when you are not directly using the app, many apps (either on purpose, or by a bug, etc) can continually keep using data.
I had this once, where had a web page open in safai, which was chaning adds (video adds) like every 30 seconds. I could literally watch my data going up and up.
Force killed safari. It stopped.
If you have apps you dont use on anything BUT wifi (you would never use while out), you can usually go in and turn off cellular data for them, that they CANT run when cellular only.
As well, as mentioned, VIDEO can be a big killer, that people are not aware of.
Sure your watching the video on whatever phone, which may be only whatever resolution.
But if your watching a twitch stream, or youtube video, which say is in 4k. It will still stream to you in or close to that 4k, its just not displayed that high. Those videos will use up MUCH MORE data.