Creating or summoning creatures is a class feature...
Polymorph says:
The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast...The creature is limited in the actions it can perform by the nature of its new form
Your game statistics generally include the actions and abilities granted by your class features1, so you would not be able to cast a summon spell, gain a beast companion, or have your tinkering birth a steel defender while polymorphed.
...but their use and continued existence is not.
However, once you have already attracted, summoned, or created your companions while in your true form, their continued existence is not a class feature. They certainly do not vanish while you are polymorphed or become inert.
Ryan C. Thompson, in his excellent answer here, explains why rage cannot be used while polymorphed, BUT Arcane Ward can, even though it is just as much as class feature as rage. He writes, "once active, the Arcane Ward exists as a separate entity...with its own pool of hit points and its own behavior". The same consideration applies to the creatures in this question. Once they are gained but after you polymorph, they remain on the field and are just as disposed to serve you as before.
You probably can't command them...
Typically summoned spirits, as well as your ranger companion, obey your verbal commands, and issuing these commands does not require an action - check the specific feature descriptions. But, while polymorphed, you:
can't speak, cast spells, or take any other action that requires hands or speech.
So you will be unable to issue verbal commands once changed. You could certainly, prior to your transformation, preload them with complex and contingent commands, with the only limits being your forethought and their intelligence.
The steel defender is a different story, though, since:
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.
Here there is need for a DM ruling. While you command the defender, this feature does not specifically say whether your commands are verbal, or hand gestures, or something else2. Further, we don't know whether the bonus action to command the defender is a class feature you have that is removed from your stat block when you polymorph, or whether it is instead provided to you by the defender regardless of your form. For consistency, a battle smith probably can't command their steel defender once polymorphed, but the specific language used in the feature could permit it, if the DM was willing.
...but their attack bonus is probably fixed.
Suppose you have summoned, attracted, or created your minion, and carefully preloaded it with a command like "after I transform, attack any target I am attacking" or "attack the source of any ranged attacks on our party's casters" so that it can continue to operate once you have polymorphed. What then? Does its attack bonus change when you change your form?
In my answer to this question, I argue that while the Summon [Creature] spells do not specify whether the attack bonus of the creature changes, all the other variable features that they have are specified as being set when the spell is cast. It makes sense that the basic power (attack bonus) of the creature would also be "built-in" when it was summoned or created, and similar reasoning applies to this question as well3.
Caveat: While my answer to that question is accepted, Thomas Markov's answer, arguing the opposite, is more highly upvoted.
1 Note that polymorph does not explicitly say that it prohibits the use of class features, and while you do have the statistics of the new form, you retain your alignment and personality. Thus it might be possible to use some of your class features, provided they: (1) are driven by alignment and personality rather than spell slots or other features found within stat blocks, (2) do not cost an action to use, and (3) do not involve speech, spell-casting, any action that requires hands or speech, or are otherwise be limited by the nature of the beast form.
2 Similarly, an artillerist artificer can activate and direct their eldritch cannon, and designate its targets, but we are not told explicitly how that happens, and whether it relies on speech or hands.
3 The linked question is about what happens when your spell attack modifier changes because you pick up and put down a wand of the war mage. It doesn't consider polymorphing yourself to change your stats. But note that one of the variable traits of a summoned creature is the Whispering Aura of the Star Spawn aberration, whose save DC is set as your save DC. If you polymorph, you no longer have the Spellcasting ability or a spell save DC - so what is the DC of the Whispering Aura? At a stretch, it could be calculated like the DC for a spell cast from an item when the wielder is not a spellcaster, but summoned creatures are not items. I think it makes more sense for this to be set at casting as well, rather than the aberration to suddenly weaken just because you changed forms.
