The simplest variant is to use the "Share a Youtube video" function. The function can be found at the bottom right after clicking on the three dots. Obviously, however, this only works with videos that have been uploaded to Youtube.
The second variant requires an installation, but works with all media you have on your computer.
Image: You use the screen sharing of Jitsi to show the video image (either you share a program window, a tab in the browser or the whole screen).
Sound: If you just play the video now, the sound will naturally come from your speakers (or headphones). To route the audio signal directly into Jitsi you need additional software. Under Windows Voicemeeter and under Mac Loopback (fee required) or Soundflower (free!).
Under Windows you now change the input of Jitsi to the microphone to "Voicemeeter Output". Now the sound of the video should end up in Jitsi.