How would you trace this curved corner for a desk?

Make a full-size template before cutting the desktop. Cardboard is cheap and easy to trim, while thin sheet material is sturdier if the shape is awkward. Fit it against the wall, trim where it catches, and repeat until the outline sits the way you want. Then transfer that outline to the desktop.

Leave a little room for fitting and sanding rather than trying to get a perfect wall match on the first cut. Walls are often less regular than they look.

If the curve is truly part of a circle, you can work out a radius from measurements and draw it with a fixed-length string as a compass. But if the wall wanders, that method may not match it well. A template or a series of closely spaced marks will follow the actual wall more reliably.

For a less exact shape, you can mark several points along the curve on a template, connect them with short straight lines, and smooth the cut edge after checking the fit.

For the support, think about the whole structure, not just the corner joint. Legs or drawer units can carry the outer edges, while a sturdy apron or rails underneath help spread the load and limit flex. If two desktop sections meet at the corner, brace that joint underneath as well. Keep any support out of the chair space, and check that the pieces can be firmly attached to the base.