Do not put footage that is being used in a project into the trash. Once your project loses access to it, the project has no way to reconnect to it later.
I don't have experience managing the trash. It isn't something I've had a great need for. There are a couple of ideas you should be able to use.
Each clip from an event that you have used in a project will have an orange bar on the lower part of the sub-clip you have used. If a clip doesn't have that bar, then it isn't being used and should be safe for deletion.
You can hi-lite pieces of clips (and whole clips) in events as rejects and then trash just the rejects. You do that be displaying only the rejects. A button appears that says 'Move Rejected to Trash'. This process might take a while because iMovie has to do some work of extracting only the rejected part of each clip and saving the non-rejected part of each clip involved. Also note that a reject hi-lite can cover a used hi-lite, so don't over do reject lengths.
If you have completed your project and done your final exports, then I think removing the project first will de-hilite clips in the events that were used. Now that the event clips would no longer be hi-lited, you'd know which ones are safe to remove.
If you are trying to recover space on your main startup drive, then there are ways to move your events and projects to another drive. The other drive will of course need to have plenty of space to hold the content.