How does such an USB step-up 5V-12V work?
As mentioned in a comment, here is an interesting video giving details about the internals of a step up converter.

I bought this item, and here are more details:



Internal:



Apart from some resistors and capacitors, there are:


a Schottky diode SS24
a 3R3 power inductor
an unknown AL877 6-pin chip (?)




I also bought this item ("MT3608 DC-DC Step Up Boost Converter Power Supply Module 2V-24V to 5V-28V 2A"):



I tried to power it with:


1 AA battery (1,35V) : non working, the output stayed always at 1.21V
2 AA battery (2.7V) : working great, the output can go up to 27.4V !
3 AA battery (4V) : working as well


During my test, there was a cool feature (I don't know if it's common or not, or which chip is responsible for this): no matter the input voltage (2.7V or 4V), the output voltage only depended on the position of the potentiometer. Ex: I set the out voltage to 9.00V when using two AA batteries.
Then I plugged 3 AA batteries without moving the potentiometer. The out voltage stayed at 9.01V, this is interesting!

I wonder if there exists step up converters able to convert 1 AA battery (let's say 1.4V) to 9V, I haven't found any in the same micro-size PCB.



Interesting reading: https://www.analog.com/media/en/technical-documentation/application-notes/an19fc.pdf