Ronaldo, Marcelo, Kroos, Modric, Bale, Isco, Benzema, Carvahal....their team is just full of good-to-great players.
Look at how they (eventually) dealt with Liverpool's high press. All of them are comfortable making quick first time passes which negated the press. All of them are comfortable in tight spaces. Look at how they set up their team to attack. Their centre backs are comfortable on the ball so you didn't see Kroos or Modric having to drop deep to receive the ball. They have at least 7 players involved in attack. United, in comparison, only have what, the front four, maybe Pogba, involved?
TLDR; They're just much much better than United. Players' technical abilities, the way they're set up, the mentality etc.
We're so far away, having spent 100s of millions and it's bloody depressing.