I think it has a lot to do with getting some brand new players integrated into the squad. Barcelona was playing like a fully squeezed lemon with all those oldies running around. They just have had enough, players like Pique. Gavi, Torres, Raphael and even Lewandowski brought in immense new motivation. That is one major reason why they are now again performing on another level. I think it is difficult to tell how much of a contribution Xavi is actually responsible for. He is most likely very good at communicating effectively. In the beginning I thought he might not make it, but he solved problems and got things under control.
That is true but not like the players they got are 20 years old neither, they found some young talent, like Gavi, but Lewandowski is also old, they just had players who were no longer good, Pique wasn't bad just because of his age, he was bad because he was bad, and that's all it is about.
Thankfully, they got some good players and they are doing much better now, and they spent the money in a right way to get the right people on the correct positions. They are still battling injuries a lot, people expected them to be a lot more healthy, but that's fine they are still leading the league and that's what matters, as long as they keep leading the league and win it, that will be enough.
Don't know if you follow the players or not but actually Pique is not there now he retired from Barcelona a few months ago.
At the moment Gavi is one of the good potentials for Barcelona and with Xavi Gavi is actually being honed now. Look at how the rotation has been given to this player, Xavi is now trying to get him on the wing. not only that Pedri and De Jong are currently being tried for a number of new positions even though only moving from right to left but that is a good thing in my opinion.