Ukraine is using NATO weapons, but NATO isn't fighting this war. If NATO fought Russia this war would have been over in 3 days.

Speaking of which, China wouldn't just be battling the US alone, they would have to face off against NATO, Japan, South Korea, Australia and obviously Taiwan, all while possibly have to fend off India, who may very well look to take advantage of the situation. Even in the impossible scenario that we would face down China, mano a mano the US would never suffer losses to the point where this country wouldn't be able to defend itself. We are not Ukraine and even they have plenty of weapons left to defend their country.
Ukraine has some damaged cities and villages in the east and southern parts of their country, they are not in Ruins from one end to the other, which should actually tell you a lot. Putin has lost over 30% of his combat strength and very possibly 30,000 troops in 3 months, in order to take a bunch of rubble, which used to be one city in Ukraine. That's a historic disaster against a country with no navy and barely what you would call an airforce. Do you think Putin can stand losing up to 60% of his army in another 3 months, especially when he can no longer repair and resupply his tanks, vehicles or missles? It's over, we're just waiting to see what happens next.