[RPG] Does a Steel Defender mounted by an artificer still get its actions, if commanded

In general, riders and mounts can be targeted like any other creature

The rules for Mounted Combat don't specify how targeting works for the Attack action (or any other action) because targeting riders and mounts is no different that targeting any other creature, with the exception of opportunity attacks.

Whether you can target either, neither, just the rider, or just the mount has nothing to do with their status as rider and mount, rather it depends on many other circumstantial factors, the most common of which are by far range and total cover.

From Making an Attack:

Choose a target. Pick a target within your attack's range: a creature, an object, or a location.

From Total Cover:

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

For example:

• If your attack has a 5' range and you are a medium creature standing at the feet of a T-Rex, then you can target the T-Rex, but you probably¹ can't target its rider.
• If you can see a rider peeking over a wall, but its mount has total cover, then you can target the rider with an attack, but not the mount.

Opportunity attacks

The rules for Opportunity Attacks state that:

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.

And that:

You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction.

The rules for Controlling a Mount also state that:

if the mount provokes an opportunity attack while you’re on it, the attacker can target you or the mount.

In short, a rider never provokes your opportunity attack when it moves using its mount's movement. But if a mount provokes your opportunity attack, then you can choose to target the rider instead of the mount, despite the fact that the rider did not provoke said opportunity attack.

However, this does not mean you can target a rider regardless of all other factors such as range or total cover. For example, opportunity attacks occur retroactively just before the target leaves your reach, but if the rider was never within your reach to begin with, then you can't possibly attack the rider.

1. The height of creatures and the location of a rider atop a mount are often not well defined so there is room for DM adjudication.

It is up to you and your DM, but likely it would be controlled.

Iron Defender (UA 11-12):

By 3rd level, the tinkering in your free time has borne you a faithful companion, an iron defender. This metallic creature resembles a hound, a cougar, a bear, or another four-legged creature of your choice. It is friendly to you and your companions, and it obeys your commands.

The Iron Defender has an INT of 4. The Dragon Talk: Sage Advice on Mounted Combat admits that this is vaguely defined, so rule at your discretion. According to Jeremy Crawford, most players make the choices for their mount if it isn't an important NPC-tool of the DM.

Mounted Combat and Controlling a Mount (PHB 198):

A willing creature that is at least one size larger than you and that has an appropriate anatomy can serve as a mount

While you're mounted, you have two options. You can either control the mount or allow it to act independently. Intelligent creatures, such as dragons, act independently.

The anatomy is suitable as it resembles a four-legged creature of your choice (Gnomes are size Small see PHB 37, Iron Defenders are size medium, see UA 11) and the creature is willing (see first quote).

I would rule that the Iron Defender is not an independent creature because it already moves on your initiative and enacts most of the benefits of a controlled mount:

In combat, the iron defender shares your initiative count, but it takes its turn immediately after yours. 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 one of the actions in its stat block or the Dash, Disengage, or Help action.

This is congruent with what a controlled mount does.

The initiative of a controlled mount changes to match yours when you mount it. It moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge. A controlled mount can move and act even on the turn that you mount it.