Many informal practices have not been codified into formal "Subject Headings" in library databases. This can make it trickier to find related items. Sometimes, there is formal language, but it is antiquated and/or biased.
For example, a Subject Heading associated with Favelas and Shantytowns is "Squatter Settlements."
In economics, the related term seems to be "Informal Sector."
Keep your eye on the words that are used to describe your topic as you search, and leverage them to find related research!