Those are cookies you see not plug-ins.
Root and user plug-ins are stored in: /Library/Plug-Ins and ~/Library/Internet Plug-Ins
Plug-ins or extensions are add ons that can enhancet the way you use a browser.
This is how to clear the Safari cache.
From your Safari menu bar click Safari > Preferences then select the Advanced tab.
Select: Show Develop menu in menu bar
Now click Develop from the menu bar. From the drop down menu click Empty Caches.
Thank you Carolyn!
I did clear the cache.
And after I cleared the "cookies" by clicking on Remove all website data, it only removed the ones named as cookies, do I then just leave these other "291 websites stored cookies or other data". Or remove them manually? You explained very nicely to me that even though it says PLUG-INS they are still cookies, but were not removed.
Cookies and plug-ins are two different things entirely.
Even if you Remove All Website Data from the Safari > Preferences > Privacy window (cookiees and data) cookies will be placed on the hard drive again everytime you visit a website.
To manage internet plug-ins select the Security tab in the Safari preferences window.
Click: Manage Website Settings...