It can move and use its reaction on its own, but the only action it takes on its turn is the Dodge action, unless you take a bonus action on your turn to command it to take another action. That action can be one in its stat block or some other action. If you are incapacitated, the defender can take any action of its choice, not just Dodge.
If the Mending spell is cast on it, it regains 2d6 hit points. If it has died within the last hour, you can use your smith's tools as an action to revive it, provided you are within 5 feet of it and you expend a spell slot of 1st level or higher. The steel defender returns to life after 1 minute with all its hit points restored.
At the end of a long rest, you can create a new steel defender if you have smith's tools with you. If you already have a defender from this feature, the first one immediately perishes. The defender also perishes if you die.
AC 15 - STR 14(+2) DEX 12(+1) CON 14(+2) INT 4(−3) WIS 10(+0) CHA 6(−2)
Saving Throws: DEX +4 CON +5 Skills: Athletics +5, Perception +6 Damage Immunities: poison Condition Immunities: charmed, exhaustion, poisoned
Senses: darkvision 60 ft., pass. perception 16, PB 3, Vigilant (can't be surprised)
Actions
Force-Empowered Rend Melee Weapon Attack: your spell attack modifier to hit, reach 5 ft., one target you can see. Hit: 1d8 + PB force damage.
Repair (3/Day). The magical mechanisms inside the defender restore 2d8 + 3 hit points to itself or to one construct or object within 5 feet of it.
Reactions
Deflect Attack. The defender imposes disadvantage on the attack roll of one creature it can see that is within 5 feet of it, provided the attack roll is against a creature other than the defender.