It's a bit of both, actually. Magic spells are the most common method of directing the innate magic nearly every witch and most demons have - its actually biological, whether you have a magic bile sac of not - but rituals can be used to harness the natural magic of the Boiling Isles, or for more complex spells that require more focus than raw power.

That's also part of what coven sigils are for. They completely lock down any kind of magical ability that isn't for that coven. It's a way to prevent witches from hurting themselves by learning too much.