I asked before about content types and how to associate data fields to each content type Documents meta data in SharePoint Foundation 2013 I would like to know if these fields can be searchable ? and how ?
During full crawl these site columns get indexed and an equivalent crawled property gets created.
All the data will be searchable by default.
If you want to use any of these site columns in the Search Query, then you should create a Managed Metadata Property and map it to the respective crawled property.