Questions about modifying my car’s stereo system…

Is it possible to connect a sub and amplifier to the head unit in my car?
When I change vehicles, where can I get reliable info about modifying my new car’s stereo system?
I’d like to connect a subwoofer and amplifier up to the original head unit that the car originally came with.
I pulled the head unit out to take a look at it and realized that it doesn’t have the wires that most cars have.
Some units have two plugs that connect together and looks like there’s no sub output — but I’m not sure.
Is it possible to connect a sub and amplifier to my head unit? If so, could someone please tell me how…?