I use only one - every magic comes from the Essaence.
Well in much the same way, so do I.
Ambient mana is the natural flow of energy for the planet/plane you're on.
Personal mana is the Ambient mana of the caster's own body and mind.
Granted mana is the "Personal mana" of the deity doing the granting.
Fixed mana is ambient mana trapped in objects.
But it's still 4 different ways of accessing mana, even though it all ultimately has the same source.
The main difference is that, in my cosmological view, magic always uses the ambient energy to work - using the caster's own energy is a sure way to quick burnout (literally most of the time), and the gods do not provide energy to their followers routinely. The difference is in the way - the technique, so to speak - to access this ambient energy, and the bias that this technique creates on the resulting magical capabilities (spell lists accessible).
An Essaence user using a Channeling list would not use the mental icon of a god - they would use their normal Essence technique to access the energy.
Essaence matrices - power containers - are objects that slowly accumulate energy and store it. They provide a shortcut to normal Essaence aggregation, because a significant amount of energy is concentrated close to the caster, but the technique to access it remains the same for each Realm.
But that's Shadow World - where gods aren't (gods), rather very powerful aliens. I kind of like the idea of most channelers being magic users victim of a good case of self-delusion (the god gives me the power), and 'the gods' playing along to keep their influence with their toys.
To give a complete picture, the Unlife - anti-Essaence - is a completely different type of power, unrelated to the three Realms except in the way it 'eats' at the Essaence. You cannot use the Unlife for anything except pure, unadulterated destruction (of something - physical, mental, emotional, whatever). That's why most servants of the Unlife actually use the Essaence - the Essaence is much more versatile a tool. That's also why a true servant of the Unlife is only an empty shell - the illusion of existence, and nothing but a black hole inside.
And psionics are another thing entirely as well, unrelated to the Essaence and working in completely different ways (and able to work even in Essaence 'dead zones'). Closer to Mass Effect's biotics in a way. And beyond rare in the world, because while most people do have some psionic potential, the availability of the Essaence and its pervasiveness make developing magical powers much more likely - developing psionics requires wiring one's mind in a certain way, which is more or less incompatible with Essaence manipulation (only more or less, since the Lords of Essaence were able to wield both powers, but this ability is presumably lost with them).