You can copy a lesson or an assignment within one course as well as within all courses in your school. It is important not to switch between the browser tabs, but to copy and paste the course/assignment within the same tab.
To copy a lesson/assignment, go to its settings (click the "gear" button to the right of the lesson title).
Click the "Copy" button.
After that, you will see the "Paste" button. Without changing the window in the browser, close this course and go to the constructor of the target course - the one you want to paste the lesson/assignment in. Select the lesson/assignment that will precede the copied lesson/assignment and click the "Paste" button in the settings.
The lesson/assignment you copied will be added after that lesson/assignment. If necessary, you can change the order of the lessons. Once inserted, the copy of the lesson/task can be edited.
Don't forget to publish the copied lesson/task.