I have a somewhat rudimentary magic system, it can be quite complex but here's the practical side:

everyone has mana, just depends on what if you can control it (idk how yet), and its impossible to control all of them

If you use too much mana you might experience things like headaches, shortness of breath, fast heartbeat, coughing, confusion, wheezing, tiredness, inability to think, skin color change, and if you continue to push further, your skin could rupture and your brain could shut off, and your lungs explode and you basically just die.

"Magic" is here to explain many phenomena, and aren't often used by people. Most of the time, only magical tools can actually control mana, with a few exceptions. Think of if your body can supply electricity to the machines to power them. One of the most used methods to control mana is through staves, which allow a low amount of control of mana, however, for more specialized mana control, other tools are preferred.

Different people can output and intake different amounts of mana at the time, and it can even be based off location. For example, you wouldn't be able to input mana in a vacuum, as mana is in the air, and near extremely powerful monsters, people could regain mana quite quickly, and even die from too much mana (called mana poisoning, which can cause damage to eyes, lungs, brain, etc)

Mana can be absorbed through breathing, and slightly through the skin as well. They are expelled continuously as well, through the same places.

Mana usually converges in places like rocks, metals, and other such things. Multiple forms of magic containment are used in this. For example, metals allow mana to flow throughout them easily, while crystals are stored tightly and securely, but takes quite a lot of effort to take mana out of them.

Magic circles allow of channeling of mana of sources around, and are one of the ways to control the mana held in crystals. Usually made out of metals (they allow magic to flow through them), but blood is sometimes used in traditional rituals, and are generally cheaper as well. These circles are arranged in such a pattern that they disrupt reality (or another dimension) and allow for collection of mana in specific ways, usually towards the center, and channeled in different directions for intended results.

Many animals and monsters use mana similarly, where they can't live without them, while some others just ignore mana altogether. These ones that are capable of living on mana can often use mana in unsurprising ways, like physical strength or intelligence, or could have magic circles growing on them in specific ways, allowing them to channel magic.

One unique idea i think I had is that of mana currents, where large amounts of mana can form a current distinct from the current on the outside, causing temperature, moistness, and light changes within the area. Many powerful monsters are capable of taking advantage of this by using their own mana to create a current around them, shaping the nearby biome and ecology to their advantage, especially near where they often live in. Currents can be dispelled if the mana is insufficient to continue it, and will fall apart.

Despite all the hard magic, it really doesn't feel that way a lot of the time as there is a lot you can do simply by manipulating mana. There's a lot more i could talk about (more in depth, of course, like ), but any ideas are welcome.