system
Given the following post, respond as if you're commenting on Reddit:
human
[Full Metal Alchemist] How is Alchemy triggered?

In FMA, when the brothers try and resurrect their mother, their hands are already on the circle, and then they decide to begin the alchemical process like magic. Similarly, Alchemists who wear their transmutation circles/arrays on some sort of apparatus can trigger and control the alchemy at will.

Does it work like some sort of magic? Is one of the key components to utilising alchemy the human soul, maybe that's a hypothetical reason why human souls are so valuable in Alchemy? I'm not sure, I'm probably reading into it, but it really does seem like there's a magical element that's completely overlooked.
gpt
Alchemy is triggered by touching the transmutation circle with intent to use it. Different symbols in the circles have different effects. However a single circle isn't limited by the quantity of stuff it can transmute (which is how Armstrong and Mustang are able to create walls and explosions of varying sizes). But the elemental ratio has to be there or else you get a rebound. But that's still not all of it.

Each person has within them a "Gate of Truth", it is that gate which allows for the performation of Alchemy. Not everyone knows of it, and only those who have performed human transmutation have seen it.

Circles are necessary for **all** transmutations. Even Edd's "clapping" counts as a circle. What happens when you see the Gate of Truth is you get all the knowledge of the world crammed into your skull at once (not that you'll remember any of it at more than a subconscious level, since the human brain can only hold so much). What happens when Edd claps is that his arms form a circle for an instant, as he visualises what he wants to transmute.