Architecture students at the École des Beaux-Arts in Paris studied the proportions and the ornament of Greek and Roman buildings, and they became masters at rendering them in ink and watercolor.
The winners of of the Prix de Rome competition were sent to Rome at government expense to do extensive studies of famous monuments. They periodically sent back drawings showing ornamental details, reconstructed facades, or perspective scenes of famous vistas, like the Forum or the Acropolis.
Artists adhered to particular conventions for representing architectural forms. The light had to come from the left at a 45-degree angle. Values lightened systematically as they went back in space. (Above: Constant Moyaux, Below: Normand, 1852)
Students developed concepts in relatively loose sketches. Then they began the presentation drawings in line, usually in grayed-down India ink, with a series of very light washes of tone to build up the modeling gradually so that the tones seemed to reside in the paper rather than being superimposed over it.
There’s an affordable book from Dover that reprints in black and white a famous set of prints assembled by Hector d’Espouy, link.
Wikipedia on Beaux-Arts Architecture, link.