I am confused when to use "in demand" and "on demand" in a sentence. Pls. differentiate

and use in sentences.

Thank you so much for the help.
In demand (extremely popular):

Lady Gaga is so popular that she is booked solid for the next two years. She is very much in demand.

The latest model of the Iphone is so much in demand that units are back ordered for months.

On demand (means whenever you want it, or whenever someone asks for it):

The contract states that we have to pay on demand. (Whenever they ask for it.)

Our cable TV company offers movies on demand. (Whenever you want a movie, you can get it.)