There we go it says it there
So Downwind is when you are flying next to the airport preparing to go Right/Left Base and the you will be on final. Extending you Downwind is when you continue on straight waiting for the ATC to call your Base. Correct me if I’m wrong.
You will be flying parallel to, but in the opposite direction, of the runway, whether on the right or on the left, which is called flying right-downwind or left-downwind. Extend downwind will then mean that you continue flying in that direction before turning towards the runway for landing (ATC - turn base), Search for this topic on the tutorials section.
Takes some practice! Learn and practice!