That post helped me figure out the difference between custom search forms and the keyword search setting, thank you!

However I still don't manage to set up custom fields for search. Here is the exact places I've tried (I'm on v9.0.0-RC with all official plugins updated):

• In Studio > Forms > Search Forms > Persons Search:
  "Edit search form" only allows to change the name of the form
• In Studio > Forms > Search Fields > Persons > Persons Search:
  There is a fixed list of the two fields "Real name" (of type `text`) and "About me" (of type `textarea`). The two items can be (de-)activated, and rearranged, but I can't find how to add new ones.
• However, in Studio > Forms > Fields > Persons > Full View Person:
  I have set up many more fields of type `text` and `textarea` with no visible configuration difference from "Real name" or "About me". The are active, visible to "Anyone", and assigned to module "Persons" (although I also tried assigning some to module "Custom").
• I also tried going through the Developer plug-in, which lets me add new search forms, but I can't add fields to those, either. If I create a new search form for module "Persons", it is empty with no search fields. If I click the Reset button, the "Real name" and "About me" fields appear, but from then on it's in the same state as default Persons search. I can deactivate the fields but no longer remove them, or add others. (As a sidenote, it seems to be currently impossible to delete again search forms created in Developer mode).

Sorry if I'm missing something obvious, I appreciate your efforts to help!