How can I make the internet video streaming quality to drop systemwide when I am approaching the traffic limit?

Say, I have 50 GB traffic limit per month. How can I do so that when I have spent 45 GB, the Youtube and other video streaming to reduce to 240p systemwide?

As far as I know there is no setting within windows to reduce your data usage per app or site nor of reducing video quality at a given point of usage. However, you can set your data limit and Windows will notify you when you are nearing your limit. see screenshot

For youtube videos you can also install the Youtube Enhancer extension to either Chrome or Edge and set it to lower streaming for all videos to 240p.
This won't affect any streaming but Youtube, though.

Other streaming providers like Netflix also have their own methods to reduce bandwidth usage. You would have to search for instructions for whatever streaming app you use.