> Someone(s) to agree to sell some land

If you rezone some area, there are already going to be some number of units on the market to begin with. Then add developers willing to pay more than the current market price because it has become profitable and you get even more.

> Usually several adjacent plots to have enough space to build - get them all to sell

This is by no means necessary. You buy one plot of land, replace a single house with a 5-story condo containing 10-20 units. The whole idea is that you don't need a lot of land but rather taller buildings.

> Planning permission

This is the same premise as zoning.

> Profit in the proposed development

If you can buy a unit for e.g. $500,000 and build 20 units that each sell for $350,000, that's likely to be profitable.

> The buyer

The problem we're trying to solve is the huge amount of unmet demand for housing. If you ran out of buyers the problem is solved.