In the SI, these units are derived from other units. For example, an area is a length squared. Of course, it is possible to proceed the other way round, for example to define an area as the base unit, in which case a length would be the square root of an area. That looks more complicated, but it's possible. Thus, whether a unit for a specific type of measurement is a base unit or a derived unit really depends on the system of units selected.
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It is area and density.
Area is length x length, or length squared.
Yes, area is a derived quantity.
The [human] population density.
Length is fundamental, area is derived.