Explosion Chain question // 3.10 - Ice Shot Build - Deadeye Ranger - Delirium PoE 2020

What gems are this ranger using to get the Explosion Chain at main attack??
Says nothing in the build only ice shot fork and the aura's at end game gems.



Video
youtube.com/watch?v=YsiT8vsxeU0&t=301s

Build
https://www.poebuilds.net/ice-shot

Please help, i want that explosion satisfaction !!

And herald of ice is active