Mobile Garrison

Once a player announces that they are activating a crew ability, no player may take other actions until the ability has been paid for. Notably, players can't try to stop the ability by changing a creature's power or by removing or tapping a creature.

Vehicle is an artifact type, not a creature type. A Vehicle that's crewed won't normally have any creature type.

You may tap more creatures than necessary to activate a crew ability.

Each Vehicle is printed with a power and toughness, but it's not a creature. If it becomes a creature (most likely through its crew ability), it will have that power and toughness.

You may activate a crew ability of a Vehicle even if it's already an artifact creature. Doing so has no effect on the Vehicle. It doesn't change its power and toughness.

If an effect causes a Vehicle to become an artifact creature with a specified power and toughness, that effect overwrites the Vehicle's printed power and toughness.

Once a Vehicle becomes a creature, it behaves exactly like any other artifact creature. It can't attack unless you've controlled it continuously since your turn began, it can block if it's untapped, it can be tapped to pay a Vehicle's crew cost, and so on.

When a Vehicle becomes a creature, that doesn't count as having a creature enter the battlefield. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters the battlefield won't trigger.

Creatures that crew a Vehicle aren't attached to it or related in any other way. Effects that affect the Vehicle, such as by destroying it or giving it a +1/+1 counter, don't affect the creatures that crewed it.

All attackers are chosen at once. You can't attack with Mobile Garrison, untap a tapped creature, and then attack with that creature.

Untapping an attacking creature doesn't remove it from combat.