Does multiattack count as one "action" that does individual attacks, or is one attack that does several damages at once?

Question

When a monster multiattacks, do you roll to hit for each piece of the multiattack, or just one roll to hit with all of the pieces?

If a rogue uses Uncanny Dodge, would they take half damage of the entire multiattack, or only one piece?

Uncanny Dodge: Starting at 5th level, when an attacker that you can see hits you with an attack, you can use your reaction to halve the attack’s damage against you.

If a player has Sanctuary on them, would the monster have to roll DC WIS to attack with each individual piece or just once to hit with the whole multiattack?

Sanctuary: You ward a creature within range against attack. Until the spell ends, any creature who targets the warded creature with an attack or a harmful spell must first make a Wisdom saving throw. On a failed save, the creature must choose a new target or lose the attack or spell. This spell doesn’t protect the warded creature from area effects, such as the explosion of a fireball. If the warded creature makes an attack or casts a spell that affects an enemy creature, this spell ends.

Answer 1

From page 11 of the Monster Manual:

A creature that can make multiple attacks on its turn has the Multiattack ability.

So Multiattack is an action that allows a monster to make multiple attacks; that's the whole point of it.

If you look at any monster that has Multiattack, such as the Aboleth (first monster in the book with Multiattack) its attacks will be listed individually, and then Multiattack will say something like:

Multiattack. The aboleth makes three tentacle attacks.

So even without the general rule, the monster's statblock usually specifically states that Multiattack is made up of multiple attacks.

This being the case, the Rogue can only Uncanny Dodge against a single attack, but on the other hand, the creature has to make a Wisdom saving throw every time it wants to attack someone protected by Sanctuary.