How to find a set of files containing a certain phrase using MacOS graphical user interface
There is a 3rd party app named EasyFind that has a GUI interface and allows you to search for strings inside files. You can search your whole drive or a single folder. Click the settings button at the top left and select scan all files to make sure your file type is searched. It will search for a user-defined string. EasyFind is free and available from the MacApp store.