I haven't used iMovie in a long time. I use FCPX. But it should be just like storing a document on an external. In the File menu, select "open" or similar and then choose the movie file/library you want to edit. The difference is that it will now be on the external drive instead of the internal. I fired up iMovie and am confident that you use the options in this menu for this...
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See this article about iMovie "library"
moving to external drive, opening it (there) and using it as you do now.
As to "left on the storage device", if you mean short-term while you develop a final video file, yes. If you mean long-term, I'd change my thinking about that, as you'll need that fast storage for future video projects. If you fill it up with a bunch of iMovie libraries you keep, you crash into this problem again.
If the goal is to forever keep the iMovie library, get yourself a big fat HDD too and move the library to the HDD after you edit, perfect, render. This will move the library files off the fast SSD storage so you have maximum room for future projects. Then, later if you want to work on a library file again, you could bring it back onto the SSD from HDD storage when ready to edit again.
I would NOT be thinking of just leaving them there for the long-term. Long term storage should be HDD. Short-term, fast storage for video editing should be SSD.