How to implement search autocomplete for specific content types?
Create a new view of content. Click on + Add  select Autocompletion Callback and add a path/url to it.

In Filter add Type filter so only shows certain content types. 

In /admin/config/search/search_autocomplete edit the search block

For WHAT - WHAT TO DISPLAY IN SEARCH AUTOCOMPLETE SUGGESTIONS? change this to your view's autocompletion callback.

For more info read the documentation.