U.S kept mentioning about the red line if China were to send weapons to Russia.
What are the consequences imposed if this red line was crossed. Just a bunch of trade sanctions and ban from using SWIFT?
That would cause enough instant economical pressure on US that they would need to go open war in order to remotely justify it. There are better ways for US to go into war footing than over a relative nothing burger like Ukraine, which the population don't want to suffer for.
What US does would depend on how much China sends. China is already giving dual use equipment including engines for suicide drones, electronics for every type of missile, radars etc as well as NVGs, bulletproof vests etc. All of this crosses America's "red line" but without UN support or National Congress support, such red lines have no meaning to China.
Obviously if China just went all in and started sending their own drones with their operators and artillery troops into Ukraine, US would be pressed into doing something drastic in return. But short of sudden, unjustified, dramatic Chinese escalation, Washington cannot do anything except more complaints.