I have different magic systems, depending on the story.
For one world, magic is an alternate form of life, separate from cells and DNA. Unfortunately it can be very vulnerable to odd things from the physical world. Whereas our biological life is very vulnerable to certain magical things. So, as is the nature of any type of life to keep on reproducing and continuing, every so often both types of life combine.
The "magical realm" is made up of people and creatures that have combined both types of life. These hybrids show a "hybrid vigor" that surpasses either form of life by itself, which is why magical creatures have long lifespans, surprising abilities to heal, etc. Actual magical talents possessed by these creatures varies wildly; mutations pop up every so often and found bloodlines of differently talented people, mages, and creatures.
My other magical system is more based on nuclear energy...sorta kinda. It's semi atom-punk, lol. Magic has different types and half-lifes, and mages have to periodically decontaminate themselves because magic that will disrupt their bodies will build up and kill them if they don't. So magic used by mages is typically made up of unstable isotopes of various longevities. A poorly made spell can actually poison everyone around it as it decays, if it decays into a particularly bad isotope, or if different types of magic in the spell decay into isotopes that are not compatible with one another (kaboom!). Occasionally a mage will make a "silvered" spell, a spell that won't decay over time and is stable, but like the periodic table only has so many elements, there's only so many spells that can be silvered. They're rare and hard to make, and often don't resemble their non-silvered counterparts at all, because if the quick'n'dirty way of getting a result was stable, it wouldn't be a quick'n'dirty way to do it. Sort of like bees can fly, and birds can fly, but the actual physical wings and mechanics differ drastically.