1) On the Classwork page, click "Create" and "Reuse post".
2) This opens the list of classes you have. Click on the class where you the post you want to share is. NB. If you've used this before, it will open the last post you used.
3) This will list of the posts in that class. Click on the one you want.
At the bottom, you have the option of creating a new copy of all the attachments. If they are just Google Docs, Slides, etc I would copy them, but if they are things like listenings or PDFs that use Drive space, I would leave this unticked, but it does depend want you what to do.
4) Then click the "Reuse" button.
5) The assignment will be added to your class. It allows you to edit it before assigning it. Once ready, click the "Assign" button.
This adds the assignment to the Classwork page. Clicking on it we can see it has included all the attachments, without us having to add these manually.
The student will have a similar view and will have access to everything.
In this example, I've shared a full Proficiency listening test, which includes the PDF with the questions, a Google Form where they enter their answers into and which automatically corrects it, the listening MP3s, a Google Doc with the answers and one with the transcript.
This also applies to reusing materials, not just assignments.
Recommendation: I would set up a separate master class just for your materials you continually use and then a separate class with the students. You can then reuse posts from the master straight into the class-specific one. It saves you loads of time!