In Illustrator this is easy, if you can imagine it or if you copy it by drawing over the example image .
What to draw, if the pen is considered too difficult? At least one can draw easily a bunch of vertical and horizontal lines. As easily (with smart guides=ON) one can draw full circles which have the material for the arcs. The lines were drawn at first, they were placed after drawing. The circles snap easily to lines:
(Do not think I drew this from memory, I had your example as locked on my artboard)
Then combine the areas with the shape builder:
The final shape is moved aside and filled with a gradient:
The blue remnants are useless, they can be deleted.
ADD: I'm quite sure this is not repeatable in Affinity designer. There's not anything which combines areas between paths like the shape builder.
The other forbidden way was to combine rectangles. I'm sure that drawing rectangles, making an union and rounding the corners with the corner tool is the easiest way in Affinity D. I cannot make it in Illustrator because I have only a legacy version without the corner tool, but it works well in Affinity D: