Does demand side economics only apply to manufacturing For example, if investment falls then manufacturing falls. but there is no investment in secondary markets.

Wouldn't recessions cause factories to collapse but why would they affect realtors, used cars, secondary retialers etc?
When factories collapse, workers in those factories are out of jobs... no money means they can't afford to buy a used car, etc.

Update: To go back to the question in the title, consumption is also a part of the demand, and it includes consumption in services as well.