In order to record meetings, this must be allowed in the group/project settings. As a group/project admin, you can select and save the option “Yes” in the settings below under “Embed video conference” -> “Allow recording”.
Now, when entering a BBB meeting, everyone is asked whether they agree to be recorded. As a moderator (see roles and permissions above), you can click on "Start recording" at the top.
If you click on it again, the recording will be paused, but can be continued later. The recording is only finished when everyone has left the meeting or a moderator ends the conference for everyone. It will appear a little later under "Recorded Meetings", which you can navigate to via the group/project menu. The longer the meeting, the longer it can take for the recording to appear there.