The highest-value thing, and the most uncomfortable: watch them use it in silence.
Give them a task: something concrete, phrased in their language rather than yours - and then say nothing. Do not help. Do not explain the button they cannot find. Do not say oh that bit is not finished. Sit on your hands.
What you learn in ten minutes of this exceeds anything anybody says afterwards, because you are observing rather than asking. Where they hesitate, what they click that you did not expect, the point at which they stop reading, the thing they assume it does that it does not.
It is genuinely painful and that discomfort is the signal. Every urge to jump in is a place the product is not clear.
Two practical notes:
Say out loud that you cannot help, at the start, so the silence is not rude. Something like: I am going to be quiet because I need to see where it is confusing, and being confused is useful to me.
Ask them to narrate. What are you looking for, what did you expect that to do. It feels unnatural for about a minute and then people forget.
Four people watched properly is worth forty people asked what they think.