Here is the definition that creates such multi-level outlines:

(setq tabulated-list-groups
      (tabulated-list-groups
       tabulated-list-entries
       '((path-fun . (lambda (b)
                       (list (list
                              ;; Project names
                              (with-current-buffer (car b)
                                (if-let ((project (project-current)))
                                    (project-name project)
                                  default-directory))
                              ;; Mode names
                              (let ((mode (aref (cadr b) 5)))
                                (or (cdr (seq-find (lambda (group)
                                                     (string-match-p (car group) mode))
                                                   mouse-buffer-menu-mode-groups))
                                    mode))))))
         (sort-fun . (lambda (groups)
                       ;; Sort groups by name
                       (sort groups :key #'car :in-place t))))))