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.