Explain what are the main sources of work for the construction and
built environment division of idustry.

There are three primary sources of work for the construction industry:
residential housing, commercial, and government.  Each has different
characteristics and scales.

Residential housing is a sector that can be broken down into
single-family and multifamily homes, where multifamily homes are
typically apartment complexes and condominiums.  Single-family homes
are usually stand-alone buildings of relatively modest size compared
to commercial structures, while apartment complexes and condominiums
usually combine multiple units into a building and may consist of a
single large building or multiple smaller buildings.

Commercial construction is typically much larger in scale than
residential housing in terms of the size of individual structures and
is funded by businesses.  While most residential housing consists of
buildings of no more than a few thousand square feet, commercial
construction can encompass buildings of millions of square feet. 
Commercial structures are also typically much taller than dedicated
residential structures, although many fundamentally commercial
buildings combine commercial and residential activities within them. 
Railroads are another type of commercial construction, although they
can also be government construction.  Amusement parks are another type
of commercial built environment.

Government construction not only consists of buildings, but also
consists of general infrastructure.  Bridges, roads, sewer lines, and
tunnels are nearly all commissioned by governments with the exception
of those used by private railroads.  Parks are another type of built
environment typically funded by governments.

Sincerely,

Wonko

Your answers to this question have pointed out some very obvious
sources i had overlooked.

Cheers