Widgets are arranged on the page they appear on rather than in a settings screen, because where a block goes is a question about that page.
The dock down the left lists everything available, grouped by which application offers it, and each placed widget has a delete cross. Anything can go in any area — there is no compatibility list to work around.
🚨 Switching an application off hides its widgets rather than deleting them. Turn it back on and they are where you left them. While arranging, a hidden one is still drawn and marked, so you are never left with a row you cannot reach to remove.