Yes, knobs / sliders on MIDI controllers can be "MIDI Learned" to control a plug-in's parameters. To check what parameters the plug-in has, load it onto a track and click the track Automation button in the Track Control Panel (or Mixer Control Panel). Details are in the Reaper User Guide.
This approach uses "plug-in parameter automation". For some plug-ins, there is an alternative: MIDI CC messages can be statically or dynamically mapped to controls within the plug-in. This second approach is done within the plug-in itself, Reaper knows nothing about it. Details should be in the plug-in's User Manual.
And, for completeness, there is a third approach: for any plug-in you can use Parameter Modulation / MIDI Link to map MIDI CC messages (or Notes or Pitch Bend or ...) to the plug-in's parameters. Details are in the Reaper User Guide.
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DarkStar ... interesting, if true. . . . Inspired by ...
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