Copy all highlighted text in Preview
Unfortunately exporting highlights in bulk is not implemented in Apple Preview (yet).

What you can do:


Show all notes and annotations from preview in a list like this (press option+cmd+4). But you'd still need to copy them manually. With this list you just get an overview and can jump directly to them within Preview.
Try the App Highlights, which provides your desired functionality. I doubt it can read the highlights from preview though. Might still be worth a try to verify if it works for you and if you want to use it for this functionality in the future.