The way I have always viewed Charms is that it’s not like learning a regular skill. Or, maybe that it is, but it’s applied to your soul, not your body or mind. The Essence of the Exalted is not something that they command or regulate or control. It is something that literally reacts to their desires, often before they are even aware of it. Essence bends to the whim of the Exalt.
Think of it this way: you see a Dragonblooded moving through the woods, she never seems to misstep. Her feet always land on soft grass that muffles her tread. There always seems to be a perfect break in the underbrush for her to move past without disturbing it. The shadows cast by the canopy of leaves always seem to line up to provide her with shade. She isn’t consciously looking for any of this, she isn’t even consciously manipulating her environment. Yet the wood essence in the plants reacts to her nature and bows to her passage. If you pointed this out to her she would give you a strange look. This has been the way of things with her for decades of her life, she can’t remember when it wasn’t so. She could no more explain to you how she does this than you could explain how you breathe or open the fingers of your hand.
Which isn’t to say that Exalts don’t consciously improve. Essence flows within them naturally, like blood and breath and the lightning in your nerves, without their conscious need or desire, and simply reacts to their intent. It’s that Intent which is the real important part. An Exalt Intends to move silently and so their Essence bends itself to that task. However not all Intents are the same. Some things are easy for Essence to accomplish, some things are hard. You can Intend for your hands to be covered in glorious sunlight all your want, but unless you are a Solar that isn’t going to happen.
When you first Exalt your Essence is inclined to a few very simple tricks. Excellencies, essence 1 Charms, etcetera. Think of it like water flowing down a hill. It’s going to flow down by itself anyway, because that’s gravity (ie, Intent) but it’s going to tend to carve channels and banks over time where it will pass around hard obstacles and wear away soft earth and end up moving faster down these rivers than it would if you just poured buckets from the hilltop. The process of learning Charms is carving those rivers, making it easier and faster for Essence to flow in the direction of your Intent but in specific ways that are more possible.
If you’ve ever seen any Nasuverse/Type Moon stuff, I imagine Exalted Charms are much like Magic Circuits: a type of spiritual nervous system which has been carved into the Exalt’s soul structure. When an Exalt learns a new Charm he forges new pathways in this system, so that Essence will flow unconsciously in line with his Intent much easier when he needs it to. This means that learning Charms is a more conscious process than using Charms. You can think of it like muscle memory. You train and train to repeat the same motions so that when you use them they happen below the conscious level, but it’s not muscle or brain tissue you are training, it’s your soul.
As for how training occurs? It depends on the Exalt. Some focus on a certain Intent and attempt to achieve it over and over until the Essence naturally carves the required channels in your soul. Others meditate until they can see the shape of their soul and use focused Intent and willpower to etch the required channels. Still others seek out tutors and helpers who can guide them through special training regimes that are focused on creating the right kind of channels in the most efficient way possible. Sometimes Charms just naturally develop, other times it requires a deliberate effort to go against your nature to force them into your spiritual landscape.
Note that only natural charms develop naturally. You can learn Fivefold Bulwark Stance by just wanting it badly enough over time, but as a Solar you can’t learn Fire Dragon Style or Sorcery without the kind of conscious effort that requires learning how to reform your soul to force Essence to move in unnatural ways. Martial Arts works like other Charms once learned because first and foremost all Exalts were designed to be Weapons and so things that weaponize them are always ‘in theme’ so to speak.
This is why Sorcery requires a Sacrifice. Sorcery is the ultimate unnatural method of Essence manipulation. It doesn’t flow in favor of Intent, it is directed and consciously controlled using formulas or processes inherent to the world, not the Exalt. Exalted Charms were designed to be simple and flow in particular ways. Sorcery runs exactly counter to all those ways, and thus you have to basically mutilate the soul in order to get it to work at all.