Example, The steel defenders attack bonus is equal to your spell attack.

If you, the PC, are polymorphed, does that actively change your defenders attack bonus?

I don't believe it should, because if you follow that logic to its real, ultimate conclusion, you lose the "steel defender" class feature, meaning your steel defender would instantly stop existing as that stat block.

This would, also, apply to all other companions and similar things. Like homonculus, wildfire spirit,

And any and all summoned creatures such as Fey spirit, abarrent spirit, etc, etc. would suddenly, for no apparent reason, suddenly take a massive hit to their attack mods.

If a player character is polymorphed, how does that interact with their companions, steel defender, summoned creatures, etc.?

For example, if you're playing a Battlesmith artificer, your steel defender's attack bonus is equal to your spell attack bonus. If your character is polymorphed, does that actively change your steel defender's attack bonus?

I don't believe it should, because if you follow that logic to its real, ultimate conclusion, you lose the Steel Defender class feature, meaning your steel defender would instantly stop existing as that stat block.

This would, also, apply to all other companions and similar things. Like homunculus, wildfire spirit, etc.

And any and all summoned creatures such as Fey Spirit (from the summon fey spell), Aberrant Spirit (from the summon aberration spell), etc. would suddenly, for no apparent reason, suddenly take a massive hit to their attack modifiers.