So what happens when Ukraine loses? Are we at war with Russia then?

NATO can’t field an actual war against Russia, NATO countries don’t have the industrial capacity to do so. NATO has big scary tools, but not many of them, and in a protracted war where the industrial power wins Russia would win out. It would be very bloody, long, and NATO would lose, so it’s unlikely that there will be all-out war.

Russia can’t even beat ukraine.

Russia is literally fighting all of NATO here.