Tezzeret, Cruel Machinist

• WWizards of the Coast2018-07-13
  Official Ruling
  The artifact stops being a creature immediately before your turn begins. It will be a noncreature artifact during your untap step.
• WWizards of the Coast2018-07-13
  Official Ruling
  Effects that modify a creature's power and/or toughness, such as the ones created by Titanic Growth or a +/1+1 counter, will apply to the creature no matter when they started to take effect. The same is true for any counters that change its power and/or toughness and effects that switch power and toughness.
• WWizards of the Coast2018-07-13
  Official Ruling
  If a face-down permanent you control leaves the battlefield, you must reveal it to all players.
• WWizards of the Coast2018-07-13
  Official Ruling
  If an effect tries to return a face-down creature to the battlefield after it leaves (such as Oath of Teferi from the Dominaria set or Abnormal Endurance from this set), that effect returns the card face up. If it tries to put an instant or sorcery card onto the battlefield this way, that card remains in its current zone instead.
• WWizards of the Coast2018-07-13
  Official Ruling
  The artifact retains any types, subtypes, or supertypes it has. Notably, if an Equipment becomes an artifact creature, it usually can't be attached to another creature. If it was attached to a creature, it becomes unattached.
• WWizards of the Coast2018-07-13
  Official Ruling
  If a creature enters the battlefield as a copy of a face-down creature or if a token is created that's a copy of one, that copy has the same characteristics as the face-down creature (in this case, a 5/5 artifact creature with no other characteristics), even though the copy is face up.
• WWizards of the Coast2018-07-13
  Official Ruling
  You may look at face-down permanents you control at any time.
• WWizards of the Coast2018-07-13
  Official Ruling
  If the artifact was already a creature, its base power and toughness will each become 5. This overwrites any previous effects that set the creature's base power and toughness to specific values. Any power- or toughness-setting effects that start to apply after Tezzeret's ability resolves will overwrite this effect.
• WWizards of the Coast2018-07-13
  Official Ruling
  The resulting artifact creature will be able to attack on your turn if it's been under your control continuously since the turn began. That is, it doesn't matter how long it's been a creature, just how long it's been on the battlefield.
• WWizards of the Coast2018-07-13
  Official Ruling
  The Core Set 2019 set includes no way to turn these cards face up. If an effect from cards outside of this set does turn one face up, the effect causing it to be a 5/5 artifact creature ends. If such an effect tries to turn an instant or sorcery card face up, reveal that card and it stays face down. In that case, it remains a 5/5 artifact creature.
• WWizards of the Coast2018-07-13
  Official Ruling
  Tezzeret's second ability doesn't remove any abilities the target artifact has.
• WWizards of the Coast2018-07-13
  Official Ruling
  Cards put onto the battlefield using Tezzeret's third ability enter the battlefield as 5/5 artifact creatures. They don't enter the battlefield with their own characteristics and then become 5/5.
• WWizards of the Coast2018-07-13
  Official Ruling
  If a double-faced card (such as Nicol Bolas, the Ravager) is put onto the battlefield face down, you don't reveal it as you do so. The characteristics of its back face don't apply.

Rulings via Scryfall