Also in my status area (many people erroneously call it a system tray) there is an icon that when I hover over it it gives me the status of notifications and Focus Assist. When I left-click it gives more information and when I right-click it gives a few options.
I think they are what developers call Toast Notifications, as in Toast Notification and Action Center Overview for Windows 10 – Tiles and Toasts.
The likely source is from Windows Notification Service (WNS) also called a push service. There seem to be multiple ways for applications executing in a browser to show notifications. We can subscribe to notification from a website; I always say no but you probably authorized notifications because they are not honest about what they are asking. There is a way to see all such subscriptions but I cannot find that now. One thing to look for is Site permissions in your browser settings. Other browsers might call it something else. In Site permissions look at Notifications and Pop-ups and redirects (your browser might use different terminology) especially.
Also look for a task manager in your browser. Chrome has a task manager.