How does a car alternator work?
The alternator has an output voltage 13.7V. I know that each load is not a linear resistor but each load represents a certain resistance to the flow of electricity then can we say that the current depends on the resistance of the load.


Yes, for a given voltage the current will depend on the load. Note that it is a little more complex than that because increasing the load will cause the voltage to drop a little - particularly at low engine speeds. You can verify this by idling your car engine with the headlights on then then load up the system by switching on wipers, fans, rear window demister, etc. The lights usually dim a little.


  Example is car alternator has a capacity of 900W of power to give that power to load at 13.7V. Maximum output current is 65A if the load resistance is 0.21077 ohms. This all calculation are based on ohm low [sic].


Yes, if you meant to type "Ohm's Law".


  Can we calculate like this for every equipment that must support rated current?


Unclear what you are asking.


  Can we use Ohm's law to calculate the output current of the alternator?


Yes. For each load you can calculate the current and then add them up to get the total current.