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There is one case that's missing: variable input, say a USB-C PD powered device that can use anything from 5-20V despite operating internally at 12V (which, iirc, most laptops do), or automotive devices that can run at anything from 6V (motorbike) to 24V (truck, bus) while being tolerant of >100V spikes during load changes.

How do these work?

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For output voltages that must be higher or lower than the input, there are other topologies like buck-boost, Ćuk, or SEPIC. In general, DC-DC converters can change their pulse duty cycle to modify the amount of energy transferred per switching cycle, and use a closed-loop control architecture to handle changes in input or output conditions. The control loop has a very fast bandwidth (usually hundreds of kHz) and can quickly respond to input voltage spikes or load transients.

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