It’s likely a very basic modelling tool apologies but I’ve been struggling to figure it out, thanks!
There are many options, could just draw that red line and Sweep2 a cutting surface. It would need to be extended a bit to make sure it cuts through the curved outer wall, but that’s a simple ExtendSrf operation
you could use ShortPath. just click on either end, the curve will draft itself along the side surface.
but your question is a bit unclear, do you want to create only the curve or the entire surface?
if this is a circular curved side wall, and the other surfaces perpendicular to the tangency of that circular side then you could simply use Offset on the previously created curve with option through point and navigate the pointer to the other side, that will create the same curve there respectively, then simply use loft. if its not circular just use short path again on the other side then loft.