Probably out of the BOM script. If it encounters that information in the parts definitions it will extract it.
..several ways lead to Rome.
There is a python tool KiField by @devbisme, that enables the extraction and insertion of this information into all your parts of your libraries via csv files. This would take care of this problem at it’s root.
It’s also able to do this for the parts that are already in your schematic.
You’d use a spreadsheet program to edit all those fields conveniently and import that information into your project/libraries. Outside of KiCAD.
You could also edit your symbol library files manually and add/edit those fields (same for the project files.. very cumbersome unless you can script most of it, like I did 
).
Afaik the parts that are already in your schematic will miss those fields until you add them manually - one by one. There is an edit fields tab in Preferences > Schematic Editor Options
..that adds those fields to any symbol that you import into your schematic, but not the ones that are already in there (just tested with an older v5+ nightly).
Also, this setting is global, which means any part you create in the library editor after you changed something here will carry those fields as well, but not the ones already in the library.
For in situ editing there is a table-edit built into EEschema since V5 under tools:
Again, no convenient way to add a field here afaik.