On the Brutalisk and Leviathan units the shields and stab symbiote are separate abilities unlocked by the same upgrade. The Symbiote Carapace ability is only able to be autocast when a dummy buff (Carapace Taken Damage) is applied by a damage response of a behaviour on the unit. On autocast it uses a set that applies the shield buff, a buff that increases attack priority to taunt units and uses a search to supress the threat of other player units.
The second ability is an Arm Magazine type ability called Abathur Symbiote Hangar which automatically creates an external ammo unit (like brood lord) and applies two buffs to it. The first buff periodically uses an apply force and teleport effect to keep it within range parameters validated by location range type validators while the second is disabled if the caster is buried and used by the autocast validators of the ability on the ammo unit to enable/disable the stab. The ammo unit is nothing special.
Other things to note are under the Unit type actors for the Brutalisk and Leviathan are actor events that set the actor reference for the symbiote to the symbiote unit when the Symbiote Recall effect is used which is then used as the target of further actor events on the leviathan to adjust the opacity.