What I've gathered on how Magic works in Ben 10 and how prevalent it is and it's relationship to Mana.

First thing first, I should point out that it's WoG (Word of God/the creators of a series) that both Technology and Magic are capable of the same stuff, and are just two different ways to manipulate reality.

Technology as anyone knows isn't just built ex nihlo, but relies on a foundation of techniques and materials discovered by people's ages past. Magic on the other hand only seldomly requires materials/cosmological events and instead relies on Mana (the settings life energy) to cast which is all sourced from one location throughout the Multiverse, Ledger Domain the realm of magic.

As said by Charmcaster it usually takes a Magus years of studying to use Magic (the exceptions being Anodites and Michael Morningstar due to having ready access to massive amounts of energy). All it seems to take to be able to cast any given spell is sufficient power and respurces if the spell/potion etc requires it. This is how Gwen was able to use the Staff of Ages, something only a Master Magician should have been able to do, and why Gwen could use the spells in the Archamada Book of Spells when even a Master Magician would need the Charms of Bezel to use.

Mana is heavily inferred to be basically Omnipresent in the universe with a few exceptions (Ectonurites and Anur Vladius, the undead planet of un life). That last one seems to point to something interesting. It would seem that Mana doesn't just extend to alien life, but planets and stars as well.

Speaking of how prevalent it is, by the time Ben is Ben 10K and known as the Hero of Heroes, it's hinted at the fact magic is just as prevalent as sci fi tech is on Earth. Which given the fact Gwen attends a school of magic in Omniverse gives credence to that.

Feel free to add stuff to this, I've been meaning to gather all info on Magic in the series. Do note I'm only focusing on the classic continuity (so the Ben 10 series, Secret Saturdays, And Generator Rex).