Setup for Divi is discussed here:

https://support.searchandfilter.com/forums/topic/divi-4-templates-blog-module/#post-226630

I think that the Post Grid module settings are slightly different now, so you may need to look further down on the first tab for the search form options.

To restrict the search to just one category, in our form settings, on the Tags, Categories & Taxonomies settings tab, ‘Include’ just the term you need.