It´s going to depend on the type, age, style and manufacturer of the air conditioning system as to how you do this. Inverter systems are going to be a massive task to do because they have quite complicated controls, it´s not as simple as switching a compressor on or off, there are many variables to control.
If it is a simple on/off style system then that is easier. Generally you will have 5 cables (or 3 for cooling only) between the units. These would Compressor live, neutral, heating valve, fan, earth. But not necessarily in that order. Some models also have another thinner pair which is an ambient temperature sensor. On these types of systems there is rarely any controls in the outdoor unit.
The controls are quite simple, a relay to switch the compressor, a relay to switch the heating valve, a relay to switch the outdoor fan. The internal fan is usually 2 connectors, 1 to drive the fan speed and one to check the speed of the fan. This is to stop the motor if the fan has seized or is spinning too slowly. The louver motors are just small stepper motors, easy to control, but you must make sure you don´t try to turn the motors too far or you will destroy the motor, the louvers can't physically turn 180º without getting jammed and the motor won't stall, it will destroy itself. That´s it for power stuff.
There are normally only 2 sensors, 1 detects the coil temperature and the other room temperature. The coil temperature is the most critical one. It has 2 functions, in cooling mode it is there to prevent the indoor unit from freezing up, in heating it controls the fan speed to maintain the evaporator (which is the condenser in heating) at the right temperature or to stop the fan when the evaporator isn't hot enough (so you don't get cold air blowing out when the unit first starts). It is also a safety device to stop the compressor if the evaporator gets too hot in heating.
Other than the IR receiver and any flashes LED´s that´s it to the simpler models. If it´s an inverter type, then forget about it, it would take all this page for me to describe how they work and even then you would have 100 questions, and it differs between manufacturers.
It would be very difficult if not impossible to do a universal controller because of the differences between manufactures, seriously they are not even similar. A lot of the chinese one´s I guess you probably could get something that might work and if you are just sending IR codes then that would be possible, but hard. Simple on/off style are pretty much extinct here now, everything is inverter and a lot more complicated.