If the app you are trying to put in picture doesn’t support pip (like dazn I found). You can start that video on another device (iPad), do screen mirror or airplay to the tv, then do pip to it and start Zwift.
I haven’t tried this, but this may work for Zwift if you want it in the small window. Do mirroring from you phone/pad, start Zwift on your phone/pad then select the pip on the tv to send the mirror of Zwift small and then start the video you want on the tv.