Only Attach A Google Form
If your Google Classroom assignment ONLY has a Google Form attached (linked) then when students submit the Form the assignment will automatically be submitted for the student.
After submitting the Google Form the student will see an option to “Open assignment.” This is unnecessary since the assignment is automatically submitted, but it’s nice for the student to confirm that it submitted and it offers the student the opportunity to leave a comment about the Google Form.
Linked Not Attached
Note that nothing is really attached in Google Classroom. We say attached, but really they are linked from Google Drive.