I've done this before in emacs but I can't remember how or which plugins were involved.
Basically you marked a bunch of files according to a file extension, and then you got asked for a search string, and finally it opened all files that matched the string as a long list of buffers/windows.
This entire list of buffers/windows was easy to modify as ordinary text after which you could save them all to save the changes.
I've searched for "search replace multiple files in emacs" but all the results seem to point to a replace behaviour that is static rather than having all the files as actual buffers.
So the question is, which plugin or command may this have been?