PreTeXtBook
Documentation about layout of Desmos interactives
According to the Desmos API:
The calculator will automatically display in one of two layouts depending on its container element's width: above (SB NOTE: or equal to) 450px the expressions list will be placed to the left of the graph paper; below 450px the expressions list will be placed below the graph paper.
We usually specify the width of an <interactive> using a relative @width attribute; it's probably worth noting in the guide somewhere (maybe here eventually, or else here, or in this sample) that if you want to see the expressions list below the graph paper, you'll want @width set strictly below 75% (since the fixed width of content is 600px and 0.75 * 600 = 450).
I poked around in the desmos api documents and couldn't find any way to exercise fine-grained control of either the width of the expressions list or the layout. It would be really cool if we could have, like, a @layout attribute we could pass to a desmos interactive element to request a top-bottom layout even if the element is wide, but if the API / iframe doesn't support that, then it doesn't support that.
If we were feeling very fancy we could self-host the API and then probably make any changes to the code we wanted. But I don't know that any of us are feeling that fancy.