There is another factor you must have missed in my explanation. This is happening on the fly if you open youtube in a new tab while you are already in a previous tab. Please read it again. You cannot hit Youtube and prevent it from affecting your login on the other tab. So clearing cache is not practical if you are using both tabs at the same time. It jumps over into the site you are logged into with you such as trying to post a video here. You would have to copy the Youtube link, close your browser and clear your cache, then load your browser back up and paste that link in your post here. That is the only way you can keep it from gathering data from the other site you are logged in or logging into. You would have to clear your cache after each visit to Youtube and not log into any other sites at the same time.
I use Firefox with Privacy Badger and Ghostery to keep me mostly clean, but an API would be able to get around them, I suspect.
If Google is using an API would they get the information from /AppData/Local or AppData/Roaming to get around any security and track me?
This seems to be more than tracking my movement around the web.
Is there an IP address or range of addresses I could block as a simple way to thwart their intentions?