Silicone rubber

Do not see an image, but for most transmission work you use a gasket and a thin layer of high temp RTV. I believe the item used in the video is just low VOC clear silicone. On the units that don't use a gasket you apply a thicker bead of sealant and tighten the bolts so the sealer just starts to flatten, then you let it cure before the final tightening. That way you don't force all of the sealant out or into the pan.