Windows 10 allows you to see exactly which processes are using data, how much, and also allow you to set a global limit.
If you find that your Windows PC is not using data, log into the modem and make sure that no other WIFI clients are connected to it, as a precaution you may want to change the password and see if your data usage stops.
Usage that high is some form of data sharing or a particularly large family. As an example, 3 adults in my house streaming movies/Netflix, twitch, uploading to youtube and gaming, basically 24-7 is roughly 350-450 gb monthly, with occasional spikes when downloading a new game that's particularly large. I'd start with making sure you don't have someone in the household downloading/uploading entire libraries of games, movies or something else, as that can add up quickly. I would also assume every streaming user in the house is going to be consuming between 150-200 gb each.
Otherwise your biggest hitters are running a website/server, network access storage or any equivalent style downloading/uploading, BTC farming or resource sharing is another big one, and all of the above but due to a nefarious program/virus or illicit access to your wireless.