When you "right-click on Applications on the Sidebar and select: Add to Dock", nothing obvious happens if you have the Dock hidden, which is the default behavior. If your Dock looks anything like mine, after you add the Applications folder to the Dock, you might not even notice it.
When I right-click on Applications on the Sidebar and select: Add to Dock on Catalina 10.15.6, and then mouse down to the bottom of the screen to reveal the Dock, the Dock now has what looks like a new Application Icon next to the Trash Basket. It does not look like a folder at all. Instead, it is a stack of icons, with the top-most being the first (alphabetically) application in the Applications folder, so at first glance it looks like just another Application on the Dock.
When I hover over it, a label "Applications" pops up over it and when I click on it, it expands into a grid of icons representing all the Applications in the Applications folder. So that is the Applications folder in the Dock.
If I then drag the Applications folder from the Finder to the Dock, it disappears when I release the mouse, because the Applications folder is already in the Dock and you cannot have it duplicated in the Dock.