How do power adaptors provide adjustable current to devices?

I answered this SE EE question a few hours ago.

My answer

In one of your comments, you (OP) said: "The power adaptor doesn't have a magical changing resistor inside it."

But the power adaptor does have a "magical changing resistor" inside it! Let's see what this "magic" is with the help of a simple electrical experiment. It is so simple that it could have been carried out in the middle of the distant 19th century (for example, by Ohm and Thevenin:-) without knowing what a "transistor" or "negative feedback" is. We can call it a "man-controlled power adapter".

To make it, we need only four elements - a simple voltage source V, a variable resistor R (rheostat) in series, a voltmeter at the output and, of course, someone willing to turn the resistor wiper. Ok, I agree to do it for free and with pleasure:-)

We can load the "adapter" by another variable resistor RL (another rheostat) in series to R. I assume you don't mind turning its wiper? If so, let's get started…

At first glance, this arrangement does not make much sense because it turns out that we simply decrease a voltage through the voltage divider R-RL. But when you decrease the load resistance RL, I will start decreasing my resistance R (and vice versa, when you increase RL, I will start increasing R) until the voltmeter reading is restored.

As a result of this "magical changing", you have the illusion that the adaptor resistance is zero. In fact, only its "differential resistance" is zero, and its "static resistance" is what it is.

With this story I wanted to not only explain how to make a constant voltage source using the principle of negative feedback but also show the power of simple circuit concepts through an intuitive explanation.

If this approach appeals to you, you can find many such circuit stories in my Circuit Idea wikibook and in this blog.