市街地 and 街並み: an urban area and its streetscape
Compare 市街地 as an area with passing vehicles or incoming smoke and 街並み as a view observed or preserved, identifying the object of each action.
市街地 names the area of movement or incoming smoke
「市街地では朝から配送の車が行き交う。」 describes delivery vehicles moving through the city streets from early morning. 市街地 names the urban area of that movement, and 朝から provides its starting time.
「山火事の煙が市街地にも流れ込んだ。」 says that wildfire smoke drifted into the urban area as well. 市街地 is the area receiving the smoke, and にも adds that it reached there too. This report does not determine the condition of the whole area.
街並み names the streetscape viewed or preserved
「丘から赤い屋根の街並みを見渡した。」 describes looking down at a red-roofed streetscape from a hill. 街並み names the view, with 赤い屋根 identifying its red roofs. It does not make every part of every building red.
「古い街並みを残しながら道路を整備した。」 says that the road was improved while the old streetscape was preserved. 街並み is the view retained, while 道路 is the object improved. ながら connects the two actions as taking place together.
Separate the receiving area from the preserved view
In 煙が市街地にも流れ込んだ, 市街地 names the area into which smoke drifted. In 古い街並みを残す, 街並み names the streetscape retained. Connect area and view to the different objects of incoming smoke, observation, and preservation.
The name 市街地 does not by itself supply population density or administrative boundaries. Preserving a 街並み while improving a road describes the view and actions in that sentence; it does not establish the whole area's condition or landscape preservation rules.
What do 市街地 and 街並み name in 煙が市街地にも流れ込んだ and 古い街並みを残す?
Show the answer and why
市街地 names the urban area receiving smoke, while 街並み names the preserved streetscape. One is the area of incoming smoke; the other is the view retained. The smoke report does not judge the condition of the whole area, and the preservation example supplies no separate legal standard.