how exactly do i distribute assets to keyframes? (osc community?)

In "Webzforevz" "HOW TO ANIMATE LIKE BFDIA | Character Animation Tutorial" video on YouTube, at about 30 seconds in, he references selecting all items and then converting them to symbol, which isn't a problem but then also has them all individually selected whilst inside the symbol? im not sure how that would work, if anybody could help it would be greatly appreciated! (i do have most animation figured out, if someone could help, id love to put you into the credits of the first episode, im the script writer and animator all at once so it may take a while, but id like to credit for helping! :D)