This is a delicate and interesting sketch I came by from David Pizarro.
Considering the use of lines in his sketch, he carefully used line thickness to demonstrate the idea I believe in Elevation view. It seems like an ultra thin point marker or pen been used to sketch the external lines and maybe pencil for inside ideas. What form the sketch fascinating is that it looks 3D and feels that the interior is standing out and mostly the blue shade either. Possibly he used tracing paper to trace the interior and then the outside lines and his charcoaled finger to create the background atmosphare. This is very cool and exciting for me to analyse this cool sketch as I am just guessing and observing the sketch by looking at each part carefully. Not sure if the lines are sketched by ruler or not but it seems they are carefully lined next to each other in specific distance. I feel he locked his wist and elbow into a comfortable angle and only moved his arm while sketching as it looks so soft and nicely rendered. The cross hatching technique been used in corners of the interior to indicate depth and angle for inside. Shading on the floor with more pressure and use of weight on rendering tools he used, created depth and made the sketch 3D as it feels like the interior or this space is floating. Corners of lines are carefully touched to each other. Therefore, there is use of intersect of lines in corners of the flooring sketch which again is another technique been used.
Finally I am immensely inspired by this sketch. If I would re-sketch this, would use tracing paper and draw in layers and make it as an animation sketch as it is very carefully designed and absolutely outstanding.
Well done David Pizarro. I wish you all the best and luck in future sketches and projects.