Borough Streets are placed on a tile adjacent to the city it is to be built in, and upon completion provides not only the listed bonuses on the new city tile, but adds any surrounding tiles that aren't occupied by another city tile to the city's area of exploitation.
Borough Streets can't be built on Ruins, Minor Faction villages, Ridges, Sea/Lake, or tiles outside the city's region, but they can be built on everything else, including Watch Tower Foundations or Resource deposits.
Borough Streets are unlocked for construction in a city for every 2 population in the city, not including bonus population from pacified minor faction villages, and the Industry cost to build starts at 150 , and increases by 150 for each after the first. For the Necrophages, boroughs are unlocked for every unit of population, and have an initial cost of 100 which increases by 100 for each after the first due to their "Cellulose Mutation" trait.
Borough Streets, like other districts, can be leveled up by being surrounded by 4 other districts of the same level or higher.
In order for a city to grow in a controlled manner, new fields must be added in a way that permits regulated and sanitary growth. By building streets, gutters, and sewage systems, cities can expand without the attendant problems of disease and congestion.
Be careful when you place your initial city - the City Center and each District needs 4 adjacent Districts to level up to level 2. This will triple the City bonus of Science, Influence, and Gold, as well as give +15 Approval for that City. Therefore, you need to plan out how the city will grow in order to maximize the benefits of a large city.
Happiness management is very important - each level 1 Borough will reduce the approval on the City by 10. Therefore, if you expand ineffectively, you can hurt your overall productivity. Using boosts (specifically Wine) can buy you enough time to build enough Boroughs to upgrade some of your Districts to Level 2.
Building in 6-hex triangles or larger can quickly bring approval back under control, however a more effective strategy is to build in double-thick lines. This minimizes Level 1 Districts while maximizing Level 2 Districts, as only the ends are at Level 1 and the number of exploited tiles vs districts is higher. This is great for regions with long unbroken runs.