Cavalier 2000 Trans issue

How does one service this sealed unit?

I assume that what you mean is you dont know how to check and fill to the full level without overfilling. The fill cap is on top of trans, the plug for checking it is under car. The car has to be raised level. Start car and allow it to warm up, shift through gears, put in park, keep engine running. There is a plug with a 11mm head on right leg next to where axle goes into trans. remove plug to check level. if fluid comes out, it’s full, if not add fluid through fill hole until fluid runs out of check hole. replace plug, shut off engine. Done

Then if it is over 30,000 miles, you just change the fluid and replace/clean the filter.