We won't blame you if you are still feeling amazed at how many apple varieties exist, have existed, or may even exist in the future...let alone all of the ciders that could be made from them! But that's not where the complexity of cider ends, we also have to consider the incredibly complex world of the microorganisms that actually turn apple juice into cider! Actually part of the 'Fungi' family (along with mushrooms), they are the real cider makers, responsible in large part for the flavours we get to experience in cider - plus, without them, we humans would not have any alcohol!
Yeasts can be laboratory grown & then added by the cider maker, or wild; existing in our environment, ready to do such things as turn sugar into alcohol when they get the chance. Most lab-grown yeasts are just a single kind of wild yeasts, that has been isolated, and grown. But if you press apples for their juice, and simply leave that juice alone, wild yeasts will almost certainly start making it into cider! It really is that simple!
And yeasts don't only make the alcohol: as fermentation happens, gas is released, which is how naturally sparkling drinks are made (if the fermentation happens in a confined space, after the pressure builds up enough, the gas is forced to dissolve into the liquid, and then comes back out as bubbles when the container, such as a bottle, is opened).
One of the reasons a cider maker might choose to use a laboratory yeast is if they want a quick fermentation, as some can work very fast, making cider in as little as a few weeks. Another reason is control & predictability, as if they are always using the same strand of yeast, the outcome is more likely to be the same. Wild yeasts however are an ever-changing little ecosystem, made up of lots of strains of yeast, all doing different things to the apple juice, and at different times in the fermentation. The exact mix of this ecosystem will never be quite the same and will change as a cider ferments, from day to day. When we say they are found almost anywhere, we mean it; there will be millions if not billions right now on and around you! And even a single farmhouse that makes cider may have a distinct population of wild yeasts, that shapes the tastes of its cider in a specific way. So when you taste a cider that is made from wild yeasts, you're sampling a really unique expression of the place it was made, & it has an inherent uniqueness - even two different barrels, sat next door to each other, filled with the same apple juice from the same trees, will likely turn out slightly differently as the journey these wild yeasts go on, as they ferment the juice, differs.
As the saying goes, with great risk comes great reward, but while wild yeasts can hold wonderful possibilities, they are not as predictable as laboratory yeasts, and when things don't turn out as our tastebuds desire, it can mean pouring a lot of liquid down the drain...or, we end up with something not particularly nice to drink. They can also take a lot longer to ferment apple juice to cider, which means a maker has to be more patient, but which can also have some benefits in creating a complex cider... So it takes understanding & skill, for makers to get the best from wild yeasts! For those makers who really master the art of stewarding wild yeasts through fermentation, the cider that results can be unbeatably complex! There are other microorganisms that can also affect how the cider ends up tasting, but yeasts are the key ones, and the ones that give us the actual alcohol!