There are two types of Zone density in Cities Skylines: one is low, and the other is high. Upgrading towards a high-density zone can benefit you monetarily as fewer resources are exhausted, and more people are involved.
I will brief you on the Zoning Strategy and further help you with how to get high zone density.
Commercial, Industrial, Office, Residential, and municipal are five zone types that you will find in the game. The zoning strategy relies on finding roads that allow you to zone on. Fortunately, most of the streets support zoning.
Ensure to circle the streets where zoning is applicable. Zoning is restricted to four squares from the road and cannot be done on the highways.
You can also change your zoning size and add associated zoning to a particular zone. You can control the low-density zone by adding a path in between. This way, it can restrict zoning to avoid high zoning in particular areas.
Cities Skylines also lets you scale different areas with different sets of policies. Either city-wide or district-wide. Make sure to select the right set of policies for the specific area as it impacts the look of that area. While raising the tax, remember that it may generate more money but will affect the value of that area overall.
You have to find the middle spot when dealing with these constraints so that they benefit you correctly. You can also equip the high-tech housing to get more variety into your residential area.
Small business enthusiasts and Big Business Benefactor policies can be used accordingly to suit each type of business in commercial zoning. You can use let go of leisure to hinder taxes for sites that promote entertainment to boost attractiveness.
Industrial space planning can massively increase the goods produced and Filter Industrial Waste. This reduces the garbage produced by the Industrial zones significantly. These are some of the zoning strategies you can note while playing Cities Skylines.
You must first hit a population limit to get the high zone density. After hitting a population limit, you will get an option to get High Zone Density. However, I advise you not to convert the zone into a High-density Zone until you have used all the land in the Cities Skylines.
As you convert the area in the High Zone density, you will have to deal with traffic constraints a lot. However, the process is simple if you still want a high-density zone. Select the zone and choose De-Zone. After this, you can re-zone the residential or commercial areas into a higher-density version.
It is pretty easy to get High Zone Density, but try to use other spaces and create new areas containing roads, transport, etc., so traffic can be dealt with quickly.