The link from Rhodes and the link and comments from Sniper297 are very good and quite thorough answers to most of the original question.
If you want to see some additional technical details about the adjustment mechanisms on and inside the torpedo itself, have a look at the G7a drawings at uboatarchive.net. Take note of the "lugs" on the top of the "engine compartment" body and the upper tail fin; they are there to ensure the torpedo is lined up in the tube to mate with the adjustment spindles.
As for maintenance; like all other complex modern weapons, torpedoes were (and still are) designed and built to be "easily" maintained. The crew did not have to take them apart and put 'em back together again in order to do the routine maintenance. Yes, you did have to pull them from the tubes, but the handling gear in the torpedo room was designed specifically to be able to pull the fish from the tube, do the maintenance and put it back in the tube without serious disassembly.
The most frequent maintenance action was just removing, regreasing and reinstalling; every time you open the outer doors and flood the tube, you're introducing the steel "hull" of the torpedo (and all of those fittings on it) to one of its biggest enemies...saltwater. The next biggest maintenance problem was based on the type of propulsion. For the G7a you had to check the levels in the fuel tank and lube oil tank and the pressure in the air flasks. For the G7e, the primary check was the battery charge and water level in the cells. There were fittings to add fluids/air if necessary and there were also numerous grease fittings, including the steering rudder and depth rudder linkages.
All-in-all, plenty to do to keep the torpedomen occupied.
