Planning Authorities all have thresholds for "affordable housing" but it is down to the individual local authority to determine how they use it and in many cases how they define it. It is handled as part of the planning process, usually via a Section 106 legal agreement.
For example, the estate I live on has about 20 affordable houses/flats on it - these are shared ownership, some 80% owner 20% council, some 50/50 with no rent on the Council element. Most are owned by young working families and are distributed throughout the estate. Causes no problems at all.
Dwellings allocated as affordable housing will be in the planning application which is a matter of public record so you can easily find out which plots will be.
Putting all of the affordable housing in a development together is very poor planning.
Developers can, and often do, commute their affordable housing obligation and pay the local authority money to build or acquire affordable housing in other areas.