Hello! there's actually several ways to achieve this effect:
for example you could have a line be attached to two bones, and have an end move to make it spring out till it reaches a corner, activate the next line, repeat etc for every corner.
Even simpler: you could use clipping, and have the whole box already done, and use clipping to reveal it in an animation.
Clipping is generally an expensive operation at runtime, but if you're going to export a gif or video for twitch, it's not a big deal. And actually, it's also not a big deal if all your skeleton has is basically clipping and a few bones like in this case.
