Data Usage Spiked

Hi, our use of the Wifi and number of devices has not changed. Everything is on the WiFi. BUT our data usage has spiked and suggests it's about to go over 1.2.

1) How do I find out which devices/programs are using the data? Since everything is connected to WiFi, why is data getting used? How do I check what's NOT on WiFi but using data? My use of Teams, Netflix, etc. hasn't changed.

2) I've read that readings have historically been wrong. How do we verify that the data is actually approaching 1.2?

3) What are ways to lower my data usage?

For background, we only have Xfinity Wifi on the Gigabit extra plan. No TV or Phone in our plan.