Futurist Operative — ruling 1

Effects that set a creature's power, toughness, or creature types will be applied in timestamp order. The timestamp of Futurist Operative's first ability is when it entered the battlefield, not when it became tapped. Notably, this means the ability will not overwrite any effects that set its power and toughness to specific numbers after it entered the battlefield. Effects that otherwise modify its power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.

Futurist Operative — ruling 2

Untapping Futurist Operative once it's been declared as an attacker won't remove it from combat.

Futurist Operative : règles, fonctionnement et interactions (MTG)

Futurist Operative is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

As long as this creature is tapped, it's a Human Citizen with base power and toughness 1/1 and can't be blocked. : Untap this creature.

How it works : Futurist Operative

Original English text — Wizards of the Coast / Scryfall

#1February 18, 2022· Wizards of the Coast
Effects that set a creature's power, toughness, or creature types will be applied in timestamp order. The timestamp of Futurist Operative's first ability is when it entered the battlefield, not when it became tapped. Notably, this means the ability will not overwrite any effects that set its power and toughness to specific numbers after it entered the battlefield. Effects that otherwise modify its power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.
No other card is named in this ruling.
#2February 18, 2022· Wizards of the Coast
Untapping Futurist Operative once it's been declared as an attacker won't remove it from combat.
No other card is named in this ruling.