You clean the outside condenser coil with a similar process (except you can/should flush it out with a garden hose as some of the coil cleaners are somewhat aggressive and should be rinsed thoroughly off the aluminum).

Change any filters. Leave the ducts alone.

I’ve never seen anyone clean their A/C system.

Put "air conditioning cleaning spray" in Amazon. You open the plastic cover of the indoor part and spray the internal radiator.

There's stuff for cars too.

I’m confident the market for this is 99% HVAC contractors and repairman rather than consumers. It’s good to know they should be cleaned.