I think they have planned to not force their power to score. Zidane probably instructed their players to play effectively. Their target seems just to qualify to the semi-final, without forcing themselves to win again, a draw result is enough. I can conclude it based on the stats of Real Madrid today, it is much different from the first Leg. They didn't play as aggressively as in the first Leg.
Zidane's strategy was right when Madrid didn't play aggressively like in the first leg, because Real Madrid had won on aggregate, for what risky
attacking play will give Liverpool players space to score goals. And like you said Real Madrid managed to keep the aggregate in the second leg
and deserved to qualify for the semi-finals. Then the next in the semi-finals will face Chelsea, in my opinion, the opponent is not too difficult
than Madrid to face Manchester City or PSG.
I'd like to say, don't be so sure about it. Liverpool have been out-of-form for a very long time and they were not much different in the CL too. But things are not the same for Chelsea now. They've had a very nice form with Tuchel. I believe that they really have the power to make big difficulties for Real Madrid and even eliminate them. So, Real Madrid should really watch out for them.