It seems like you’re combining two questions:
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how to install a plugin. There are a few ways. You can use File->Properties->Plugins->Install and then browse to the RHP file or do it a few other ways . The much better option (which provides a free ‘uninstall’ capability) is the Package Manager. Up until the point you have a fairly complex plugin, this is probably the best way (and probably even then!).
Once a plugin is installed, it is available every time Rhino starts.
Rhino - Package Manager Guides (rhino3d.com) -
when the plugin actually loads/enables. Oddly enough, another dev and I were just discussing that last night.
How to set enabled a plugin automatically - Rhino Developer - McNeel Forum