I created a model in a file last year. I need to copy it to a new file now. how do I do it?
Select your model, CTRL+C, go to your new file, CTRL+V.
Learn to make reusable Components and save them to a local collection.
Then, you can easily bring them into new models via the Components Browser.
(In Windows) You can copy any group, component or geometry from a sketchup model to the clipboard, then close all instances of sketchup, open a new or existing sketchup model and paste it.
So as MAC for that matter.
When you are in a model, you can import another SU model using File menu --> Import ….
Make sure that the format is set to skp in the File Format pull down list.
The imported model is imported as a component so it doesn’t interfere with the geometry of the model.