On 14 May 1979 the Miami Beach Architectural District went onto the National Register of Historic Places with around 800 contributing buildings. It was the first urban district of twentieth-century architecture in the United States to be listed, which at the time was a genuinely radical proposition: the buildings were forty years old, several were flophouses, and the prevailing view in the city was that the correct thing to do with 1930s Miami Beach was knock it down and build towers.
The person who stopped that was Barbara Baer Capitman, a design writer who founded the Miami Design Preservation League in 1976 and spent the next decade arguing, lobbying and occasionally standing in front of demolition equipment. She lost buildings. The Senator Hotel on Collins came down in 1988 and became a car park, which is the loss local preservationists still cite. But the district survived, and the reason South Beach looks the way it does is that a small group of people made an unfashionable argument early enough.
The legal boundaries run from the Atlantic west to Lenox Court, and from 6th Street north to Dade Boulevard along the Collins Canal. The walkable core most visitors mean is narrower: roughly 5th to 23rd Streets across Ocean Drive, Collins and Washington, which is about twenty-three blocks of a barrier island.
Three styles share those blocks and they are easy to tell apart once you know what separates them.
Mediterranean Revival came first, in the 1920s boom: stucco in warm ochres, barrel-tile roofs, arched openings, twisted columns, wrought iron. Española Way is a whole block of it.
Art Deco is the 1930s answer, and it is the one people come for. Flat roofs with stepped parapets, horizontal banding, curved corners, glass block, vertical name signs, and the "eyebrows" — flat concrete shades cantilevered over each window, which exist because this is a subtropical city and shade is structural, not decorative.
Miami Modern, MiMo, is the postwar third act further north: cheese-hole concrete screens, boomerang shapes, cantilevered canopies over driveways.
Learn those three and the district stops being pretty and starts being legible.