Trainer in a web conference able to see what participants are browsing?
You probably had a trainer that was very good at reading faces.
Student screen-share was requested on Github in the feature
request of
Screenshare from student side #8445.
The answer there was:
You can do this now if you make the student presenter and then invite them to share their screen. See tutorial video for Moderator/Presenter: https://bigbluebutton.org/html5/.
Note that the student must initiate the screen sharing and give their browser permission to share their screen. This isn't something that the instructor can initiate.