The registration feature is available only if enabled in the virtual room settings. By default is disabled:
Registration can be started by Owner or Moderator users:
At the end of the meeting, recordings will be processed and after a few hours will be available for playback. The recording list is available in Room Recordings section by selecting the desired room.
More information is available on the BigBlueButton documentation: https://u.garr.it/bbbrecordings