Doug uses Verizon wireless for his home Wi-Fi and he's rather shocked in how much he's been using the last few days. He's only really checking email and such. Leo says that in Windows 10, under the control panel, there is a data usage tab on the Network and Internet. It'll show him how much data has been used and which app has been using it. This can help narrow it down. He should also check his browser history. And he shouldn't leave his browser open if his computer is left on.
He can also set his Windows settings to a "Metered Connection." He'll need to go into settings in the Network and Internet Control Panel. Then in Advanced Options he can set it as a Metered Connection. Then he can set a limit. He should also check his browser extensions. There may be something running in the background. It could also be a timed cloud backup. Open browsers will also use a lot of data because the advertising is cycling over time. Ultimately, the best thing is to shut down the PC.