You don't normally need to 'put' anything into a folder.
In very old versions of SketchUp there was a Plugins folder inside the Program Files SketchUp subfolder.
This had all sorts of issues with security permissions etc.
So in all more recent SketchUp versions it creates your own user Plugins folder in the AppData folder path [on PCs].
You don't normally need to access this folder directly [it is actually a hidden folder by default].
Almost all Extensions/Plugins mow come packaged in an RBZ archive format.
These are installed using the SketchUp menu item Window > Extensions Manager, 'Install...' red-button.
Pre-2017 this installer's location was in Window > Preferences > Extensions, 'Install...' gray-button
Find the RBZ and then this installer extracts the various files and subfolders needed by the Extension/Plugin and puts them properly into your Plugins folder...
Manually installing / copying plugins is a recipe for disaster, e.g. when you miss out some vital file etc.
Rarely you might have a collection of files etc in a ZIP archive - you can convert that into an RBZ by appending .rbz onto the very end of the file name - then it installs in the standard way.
If you have a lone .rb file [e.g. you are writing your own Ruby code] then you will need to access your Plugins folder to add it.
It that case use this snippet in the Ruby Console + [enter] to open that folder...