Mirror-Style Master

• WWizards of the Coast2023-04-14
  Official Ruling
  Backup confers only abilities that are actually printed below it. Any abilities that are gained by the permanent are ignored, including abilities gained due to a resolving spell or ability or copy effects.
• WWizards of the Coast2023-04-14
  Official Ruling
  If the copied creature has in its mana cost, X is 0.
• WWizards of the Coast2023-04-14
  Official Ruling
  If a backup ability targets the creature with backup, that creature will get +1/+1 counters, but it won’t gain additional abilities.
• WWizards of the Coast2023-04-14
  Official Ruling
  Although the tokens enter the battlefield attacking, they were never declared as attacking creatures (for the purposes of abilities that trigger whenever a creature attacks, for example).
• WWizards of the Coast2023-04-14
  Official Ruling
  If the copied creature is copying something else, then the token enters the battlefield as whatever that creature copied.
• WWizards of the Coast2023-04-14
  Official Ruling
  Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the copied creature will also work.
• WWizards of the Coast2023-04-14
  Official Ruling
  If a permanent enters the battlefield as a copy of a card with a backup ability or a token is created that is a copy of that card, the order of the printed abilities is maintained.
• WWizards of the Coast2023-04-14
  Official Ruling
  The abilities that backup grants to the target creature are determined only once, at the time the ability triggers. They won’t change if the permanent with backup loses any abilities before the backup ability resolves.
• WWizards of the Coast2023-04-14
  Official Ruling
  Each of the tokens copy exactly what was printed on the original creature and nothing else (unless that creature is copying something else or is a token; see below). It doesn’t copy whether that creature 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. (Basically, it won’t copy anything that made it modified in the first place. C’est la vie.)
• WWizards of the Coast2023-04-14
  Official Ruling
  If the copied creature is a token, the token that’s created copies the original characteristics of that token as stated by the effect that created that token.
• WWizards of the Coast2023-04-14
  Official Ruling
  All of the tokens enter the battlefield at the same time.
• WWizards of the Coast2023-04-14
  Official Ruling
  If a backup ability causes another creature to gain abilities, the creature with backup will still have those abilities.

Rulings via Scryfall