Franz,

Some advice I can give is to quit trying to look at the big picture. Even experts don't look at the big picture when outfitting an electric plane.

Keeping it simple:
Start with the battery. That's where the power comes from, not the motor. A battery is made up of multiple cells, each cell having a nominal voltage of 1.2V. So, when you see the term "8 cells" for example, you can easily translate that to mean "9.6V" and vice versa. Generally, electrics are referred to in terms of cells.

The battery is what provides the Volts and Amps to fly the plane. Key to powering an electric plane is figuring out how many of each you need. Once you have that information, choosing the motor is an exercise in shopping. Simply choose a motor that is rated to handle the Volts and Amps you determined earlier.

Understanding simple electronic concepts like series vs. parallel, Volts, Amps, Watts, and Ohm's Law are key to designing your own electric power systems.

The best way to learn is by doing, as Steve Lewin alluded to. Everything will be confusing until you see it all in action, hence the recommendation to start with a proven setup, just to see what goes into a successful design.