The Google Home app is used to monitor total data usage and per-device activity. If you launch the Google Home app, then select the "Wi-Fi" bubble at the top of the main screen, and then tap "Internet", it should show you things like the most recent internet speed test results, current (real time) usage in both directions, and allow you to see total usage over several time scales (under "Usage", tap the "1 day" label to change it to "7 day" or "30 day").
If you switch to the "Devices" screen, you can see all of the devices that are currently active, followed by any devices that were active recently (but aren't connected at the moment). Tap "Default" on the right there to switch the sort order to "real-time", "day", "week", or "month" to see which devices have used the most data in those timeframes.
When you tap on a single device, you can see the real time usage for that device as well as a Usage graph that can be switched between 1, 7, and 30 day windows (just like the total) to see when a device is using the most data.
I just wanted to jump in real fast to see if you saw MichaelP's reply and to see if you still needed some help on this or if you were able to get it sorted out. If you are still needing some help, just let us know and we'll be happy to continue helping.
Just one quick final check in here since activity has slowed down. We'll be locking the thread in the next 24 hours, but if you still need help, I would be happy to keep it open. If there's more we can do, just let me know.