I’m a complete beginner. What’s the shortest way to learn drawing manga?
Quite simply, you can't effectively simplify subjects and backgrounds for a comic until you have learned how to draw them properly in the first place. There is no "magic manga shortcut," with or without using technology. The reason the tutorials you've seen expect that you already know how to draw is because that is a prerequisite for drawing a comic. Just like you can't do calculus unless you understand basic math, you can't do stylized art without understanding the structure of the subject you're stylizing.

Creating a proper comic goes beyond simply being able to draw a face or a hand. A comic includes props, clothing designs, and backgrounds, so you'll need to learn to draw everything, not just a character. You'll need to learn multi-point perspective, lighting and shading, fabric draping, and more to be able to convey your setting effectively.

Beyond that, creating a proper comic goes far beyond just drawing. Where most people fail, besides thinking there's a "magic manga shortcut" and not learning their basics, is thinking that because they have read a comic or two, they now know everything there is on how to create their own. Very few people actually put the pieces together properly, resulting in a lot of very poorly constructed web comics, fan comics, and even professionally created comics! There are multiple books on how to construct a comic, for good reason. It's less intuitive than you think it is. If nothing else, you should pick up a copy of Understanding Comics by Scott McCloud, so you can start to understand how and why comics work. From there, I recommend Making Comics by the same author, and Drawing Words and Writing Pictures by Jessica Abel and Matt Madden. The former contains some basic guidelines and tutorials; the latter is constructed as a full comics-making class.

Lastly, once you've learned the fundamentals, found your style, and learned how to put together a comic, don't think you're going to create your master work on the very first try. You can't. You need to put everything together and practice. The number I've generally heard quoted is "You need to draw 200 pages before you start drawing good ones." You need to work hard to create a good comic. There are no shortcuts.