It started as some NATO member countries on the 19th of March. My understanding is that NATO later voted on it to become fully involved by the 24th and took over by the 31st. That's how NATO works - when a few members get involved, everyone else will follow.

Let's imagine a few NATO member countries start bombing Russia. If Russia strikes back, the NATO pact means that all of NATO becomes involved. That necessarily means that if any NATO member country is involved, all of NATO is involved. NATO is basically a shield for NATO member countries to do whatever they want, because if anyone fights back it triggers the whole alliance.