4th method °°: grep in the Custom View with :%grep ...
After performing a first text search in a folder, you get a list of results within a frame made of continuous lines called the menu
From menu, import the files into the custom view by pressing the b key.
The top line of the custom view looks like this: [Grep main thread] @ ~.....
Finally, to perform a second text search within the files that matched the first grep, use the command :% grep ...
°° hopefully, this fits into the discussion