How do you silence all tracks at once at the end of a song ?

One way would be to automate volume on the master track.
Another one is to create an empty track and use automation to switch "solo" mode for this track at a particular position.
If those tracks are Audio tracks, you can make sure the audio regions on each track end at exactly the same position.

Go to the mixer. right-click onto the Output (OUT 1-2) track and select to create a track in the arranger window. Now you have the OUT 1-2 in the arrangment window. Select it and press A for automation.

Go to the end of the song. Where you want to start the automation, click into the small black line of the track. A small node appears. Leave it at 0db (Like Output usually is). Now you have the start node. Now click right beside it to create a second node. Grab and drag this node down to -∞ (down to the bottom) to tell the track to cut off the volume. You can even create smooth fadeouts via the output track. Just drag the second node to the right to flatten the "ramp" between the two nodes.

To remove a node, re-click on it. To delete automation, select the track and press CTRL-CMD-DEL

Fox