In a perfect world a perfect buck regulator would be like a linear regulator but with one more component, namely L, the inductor in the circuit below.
This could reduce the supply voltage to (say) 5V just like an LM7805 would but with the added bonus that efficiency would be close to 95% i.e. it won't dissipate a lot of heat.
What you see more usually is this: -
This would be an adjustable "perfect" buck regulator - note the two resistors providing a "sniff" of the output voltage at a ratio that defines the real output level. Hardly any different to a regular adjustable linear voltage regulator except better efficiency of course.
More often than not (such as in the LMR12020) you get something like this: -
The extra components are shown with red circles. The boost capacitor helps the internal power FET turn on better and therefore improve efficiency - this is a "don't-care" on linear regulators because they have poor efficiency anyway. The external schottky diode is present because this isn't always easily implementable in some chips.
There are quite a lot of switchers that do have an internal "pull-down" FET and these are called synchronous switching regulators and are more efficient (and have less ripple voltage on the output) than those which use a schottky diode.
So "simple" is getting a little more "complex" and this is just the Buck regulator - it is only able to provide an output voltage that is smaller than the input voltage - it is a step down regulator.
All but the first example provided by the OP could use a simple buck regulator and, depending on the output current requirements and input voltage range, may need to have their inductor value changed to optimize performance.
The first example in the question is both a step up (boost) regulator and a step down (buck) regulator because to get 15V out the input voltage range is 6V (boost required) to 20V (buck required).
The LMR12020 would only give you half the circuit, namely the buck part - however there would be a problem because the required input voltage range extends to 20V and this is at the spec limit for the device. Let's say the OP would be happy with 6v to 18V on the input; this could be converted down to say 4V then you could put a boost regulator on the output to "jump" this up to 15V - but there's another problem and that is the maximum current that the LMR12020 can provide - 4V @ 2A (limit) is 8W and this power is the maximum that is available (via a boost regulator of any type) to the output (15V) - this means o/p current is more like 0.5A.
Conclusion Depending on your opinion, your skill set and your experience, you may regard this as simple or you may not.