I'm unclear on how attacks and grappling work with monsters as opposed to players. Let me elaborate my question.
With PCs, it seems clear to me. Let's say a PC has two creatures grappled and thus has no free hands. If the player attacks on its turn, it can make an unarmed strike against one of the grappled creatures (although it's not clear if it can use the hand that's grappling to make that attack: squeeze, crush, etc.).
Now in the case of a monster, it seems unclear because they have specific actions/attacks that can grapple. Can those same attacks be made even if they are holding the grapple? I'll use a couple of examples to clarify.
A Constrictor Snake has two options for attack: Bite and Constrict (which grapples on a hit). If a snake hits character A with Constrict (A is grappled) on turn 1, and turn 2 rolls around, can it attack a still-grappled character A with Constrict, or must it use Bite?
A T-Rex has multiattack with Bite and Tail, but can't use them on the same target. If a T-Rex hits character A with Bite (A is grappled) on turn 1, can it then Bite a still-grappled A on turn 2, since it can't attack A with its tail?
I'm asking because because a lot of gameplay or monster strategy blogs seem to indicate in their examples that the answer to this is "no".