If someone casts true polymorph on a simulacrum in order to polymorph it into a creature, which part of the spell is used: "creature into creature" or "object into creature"?
A simulacrum is a creature
You shape an illusory duplicate.... The duplicate is a creature, partially real and formed from ice or snow, and it can take actions and otherwise be affected as a normal creature.
Since the spell says the duplicate is a creature, you treat it as a creature for all relevant effects, including the effects of true polymorph.