load pull amplifier
1) If your RF system uses 50 ohm, then, of course, your amplifier output should be 50 ohm. But your transistor drain should see impedance ZL. ZL can be an impedance regarding design requirements such as the highest efficiency, max output power, etc.
2) You can simulate any part of smith chart, you can set it at schematic. ADS examples usually use reflection value to define the sweep are for load-pull simulations. Therefore, you need to calculate reflection if you use impedance data as reference. There are lots of online calculators from impedance to reflection or other way around.
3) Terminations of the harmonics are important. But you have to be careful. If you have a transistor model including package parasitics, bonding wires, etc., you may need a specific impedance value instead of short or open to maximize the performance. There are many studies about harmonics terminations. If you are a beginner, I recommend books at first.