Switching between full screen of speaker and computer screen of choice

[JPM001]

I saw a zoom session where the speaker started with full screen of their video of themselves...during conversation they would toggle to full screen of PPT or Chrome session (with their video in lower right).   Once done with the content they could easily switch back to full video of just themselves....    Here is a LINK of what I'm talking about https://www.youtube.com/watch?v=F56Qh5WMd5g

how is this done?