An smps / buck boost dc-dc converter is supposed to have what we call a feedback line / sense, it must lower the output voltage if it tend to goes up and vice versa to obtain what we call regulation / voltage stabilization
Same for linear psu's too
You see a lot of tl431 devices in them, normaly they are used for a voltage reference, but ive seen some starnge tricks with them
And normally for the voltage / sense feedback a good psu have an optocoupler to isolate this line to get in the end an isolated psu between the input and output
Buck boost or buck down dc-dc for most of the time are not isolated between input and output , and i try to avoid them, when they fail, the damages are more extensive ..