You don't need to keep all of it working. Most of it can fall apart. During the decline, important things will retract, move around, be protected. Look at what happened in the Holocaust. European Jewish culture wasn't wiped out. It was dealt a massive blow. Many important things were lost. But the community closed ranks and certain assets were protected too.

This apocalyptic thinking is something you only do if you're so removed from violence that you have to imagine the whole world ending to conceive of it getting to you. People who actually deal with violence know they never get all of us. People are lost. It sucks. But we are resilient too. In a million people, there are always a few who are both smart and lucky and are able to reorganize reality ahead of the wave. That's why there are 6 billion of us. We survive.

The humans that survive such an event will not be the smartphone engineers, but the subsistence farmers.

How long would smartphones be able to preserve knowledge? When you can no longer get clean power to charge them, they will cease to function. When they break, there will be no means to repair them. And most critical knowledge isn't even contained on the phone, it's on other machines which are likely inaccessible given the number of failure points in our global networks. If a catastrophe occurs, this high technology will be the first class to break. And no one person or group can recreate them; they depend upon global supply chains and expertise and manufacturing techniques siloed in hundreds of companies. And if a collapse did occur, we might have more pressing needs than recreating such technological marvels, like feeding and clothing ourselves. The value of such technology would vanish overnight if the supporting infrastructure collapsed.

To preserve knowledge you need it to be usable and accessible as the high edifice of our technological society crumbles. That would mean quality paper, vellum or some other durable material. We know it can last for hundreds to thousands of years if properly cared for. And I suspect that with the essentials of our knowledge base preserved and disseminated in such a way, we could quickly re-master the missing advanced stuff in a handful of decades. But if we relied on smartphones to save us, we'd quickly become savage and starving cavemen!

One of the more dangerous tendencies of modern times is that precision manufacturing techniques are often undocumented and are learned skills which require passing on from masters to trainees. There's plenty of technology from WW2 and earlier which we already can't recreate even though we know exactly what they look like. The people who knew retired and died. The drawings and documentation are useless without full training--we'd have to rediscover and refine the processes from scratch. Just last week I was reading about the restoration of a German WW2 fighter plane. They had to replace the entire engine because they couldn't manufacture a replacement part for the (repairable) original, because the machines to make the parts no longer exist and the expertise was lost. We'd have to make the machines to make the machines that make the part. Our technology base is built upon a house of cards. It's not a new phenomenon, but what is new is our utter reliance upon technology we have little control over to feed us and run our lives, making its loss quite disastrous.

Being on the right side of a knifes edge event (Toba example) doesn't prove we're resilient, it shows how susceptible we are to forces beyond our control, just as retrospectively guessing a coin toss doesn't mean we can predict the future, that's the anthropic principal at work. The second example shows how susceptible Civilization is.

For more examples look at our closest relatives which survived on earth longer than we did but ultimately went extinct.