1. In a Components folder at the root level of your Solution folder. If you remove the Component from all projects in the solution, it will get removed from this folder as well.
2. A copy is also stored in the following locations:
On Windows the Component cache is at:
(Full path: C:\Users\<username>\AppData\Local\Xamarin\Cache\Components)
(Full path: /Users/<username>/Library/Caches/Xamarin/Components)
So to remove a component completely from your system, delete it from your projects/solutions and from the cache folder above.