How Johann Sebastian Bach prepared 7ths or 9ths and added to them the 3rds

A great example is this:


Air - J.S. Bach
https://m.youtube.com/watch?v=E2j-frfK-y


Reference:


Instructions for playing thorough bass and 4 part harmony made by Herr Johann Sebastian Bach to his scholars in music - Bach's biography - Spitta - appendix


https://books.google.gr/books?id=sc-JCgAAQBAJ&pg=PA315&lpg=PA315&dq=instruction+for+playing+thorough+bass+made+by+herr+johann+sebastian+bach&source=bl&ots=-mdXIv8seS&sig=72Nsn7bA0omhDi6M9aqFQAMzti4&hl=el&sa=X&ved=0ahUKEwih8_TUl8XUAhVJCMAKHWgIAX4Q6AEIHTAA#v=onepage&q=instruction%20for%20playing%20thorough%20bass%20made%20by%20herr%20johann%20sebastian%20bach&f=false