Mirrorform
Each nonland permanent you control becomes a copy of target non-Aura permanent.
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Official rulings (4) · Scryfall
Because the permanents aren't entering the battlefield when they become copies of the target permanent, any "When [this permanent] enters" or "[This permanent] enters with" abilities of the copied permanent won't apply.
If the copied permanent has {X} in its mana cost, X is 0.
The permanents copy exactly what was printed on the original permanent and nothing else (unless that permanent is copying something else; see below). They don't copy whether that permanent is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.
If the copied permanent is copying something else, then the permanents become copies of whatever that permanent copied.