outskirts vs. suburbs vs. exurbs

[Ban my account ffs!]

I mostly hear outskirts to refer to the area near the edge of an urbanized area, whether it's a small town or large city.

Exurbs traditionally were long established cities that were not connected to the large, growing metropolitan area but itself began to grow rapidly as the metropolitan area expanded before eventually being absorbed into a continuous urban area.

Suburbs are generally connected to the main city in a metropolitan area and may or may not have been a town/city before they were developed fully and connected to the urban area.  In the case that they were their own community, they likely could be classified first as exurbs and then become "suburbs" as they become part of the continuous urban conglomeration.

So outskirts is more generalized while the other two have more specific definitions.