This is a graphical depiction of how I believe that PONY magic works in equestria. The following is why, and may contain some spoilers.
EXPLENATION
This is how I think magic works, because of something Tia said that doesn't make sense otherwise… youtu.be/37JV3vBrFI0?t=18
"Twilight created, new magic… proving without a doubt that she is ready to be crowned…" Now, this can't mean a new spell. Because new spells are being, and have been invented all the time. But no one ever got crowned for that. But, become an alicorn and instantly, here's your crown and title. Congrats on creating new magic btw.

There are lots of things Celestia says in her speaches that suddenly make a lot more sense if we assume that "alicorns create usable magic."

This also tracks with the latest film (spoilers) where an artificial alicorn is created before the magic gets a jump start.
(also, look at the glass mural in the film more closely… the 3 shards of light coming off the crystals are the same colour as twilights cutie mark that is actively bathing the land and pony kind in light… I think Twilight made those crystals to keep magic going when she realized there wasn't an alicorn to replace her.)

Note: I also think that Alicorns are more of a catalyst than a generator. Converting the raw unusable potential magic pony kind generates into something pony kind can use.