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The core of Extensions.
get($extension_name) - get instance of an existing active extension.
locate_path($rel_path, ...) - search full path of the file by given relative path. Will search in child theme then in parent theme then in framework. These are the places where an extension can be. By default will search in all places, you can specify only the places you want.
framework/core/extends/class-fw-extension.phpto see examples of how this method is used.
locate_path_URI($rel_path, ...)- same as locate_path() but will return URI to that path instead of full path.