I doubt if the composer of the song was a Buddhist, but what a wonderful Buddhist anthem. When you have freed yourself from the control of your ego-mind, know that you have everything you need inside yourself to be at peace and happy, and so you do not need anything external to be in a particular way, you know that you will be ok, spiritually safe, regardless what life throws your way because you have returned home and will always return home to your true Buddha self. And so you can truly say, que sera sera.
But what a challenge it is to get to that point in one's practice. Most people don't accept the present as being the way it is. And they certainly don't feel that they will be ok in the future if things don't turn out the way they would like them to, and so they don't accept that possibility. They are attached to every effort they make regarding their future; they do not have the attitude - if it happens, great; if it doesn't, that's ok too. And so they suffer from fear, anxiety, and endless frustration,
This is unfortunately true for many practicing Buddhists as well, partly because they have not received teaching that goes to the root of suffering, that "scratched the itch," as 17th century Zen monk Master Bankei said. Partly because they either don't really have the intent to free themselves from the control of their ego-mind, they don't want to give up their judgments, cravings, and attachments, or if they do, the ego-mind is so powerful that it overwhelms their intent and their faith,
If you want to get to the point where you can truly say, que sera sera, try watching my video series, "Coming Home." That will help get you started. It's a challenging task to overcome everything we've been taught about interacting with ourselves and the world around us.
The ability to free yourself from the control of your ego-mind impacts everything in your life that causes you suffering. I have been posting and answering questions on Quora for the past several months. And to virtually every troubled question I am asked, the answer is the same, free yourself from the control of your ego-mind.
The serenity prayer ends by asking to have the courage to change the things we can - which is how we relate to ourselves and the world around us, the thoughts we think, the words we speak, the actions we take. That is what freeing yourself from the control of your ego-mind is all about.