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I recommend PluginBoutique because of their points and all the free stuff they give away. And remember, never pay full price for plugins. The sale prices are the actual prices, and they go on sale regularly.
Watch some kenny vids on this or something first.
Plugin installers will need to know the path of your VST folders when you do the install. These are configurable in Reaper’s options, you just need to install the plugins where Reaper will look for them. They will be available in any DAW that knows the plugin directory location.
Install both the VST and VST3 versions of all plugins. You can optionally not install the audio units and pro tools formats if you like.
Then to use them in reaper, you need to add them to the track FX of a new track and set it up for MIDI. Reaper has a shortcut for this (Insert new track with virtual instrument).
I’d watch some “using virtual instruments in reaper” vids first.
And yes, there’s virtual instruments for all sorts of instruments; most common are drums and synthesizers but there’s also piano, guitar/bass, sitar, harp, harpsichord, whatever. Most synthesizers will have multiple patches emulating instruments as well.