Spontaneous spheres are a very simple answer to this question - we don’t know!  They simply appear, and may not even be easily differentiated from a different spontaneous sphere unless there is a difference in the settings.

Created spheres, however, have a number of ways they can appear.

Most common is they come to be through the desires of an individual, whether consciously done or not.  Say a lockette hates technology with screens - over time, their apartment may transform into a sphere distinct from the larger sphere it is in, where technology with screens cannot enter.

Some institutes of higher learning have codified different rituals with which to create new spheres.  While they use this primarily for research, there are copies of these rituals rumored to circulate online and by the hands of magical dealers.  Of course, there are plenty of fakes around… but just because a ritual is not authentically from one of these institutes does not mean it cannot wind up working to create a Sphere if the lockette possesses enough power and belief.  Magic does not care for the origins of what you do, only that you do it.

Rituals typically vary - there are instantaneous rituals, meant for experienced and powerful mages (and typically meant to create small spheres that may grow with time).  There are medium-length rituals (ranging from weeks to months) of actions repeated at regular intervals (days, weeks), meant for inexperienced or less powerful mages (still meant to create small spheres that may grow with time).  Finally, there are long-term rituals, performed over years - or even centuries - meant for patient and experienced mages, regardless of power.  These long-term rituals may create vast spheres, if enough has been poured into them over time.

Still, an individual does not need a ritual to create a sphere.  It can happen entirely by accident - but because there is a creator, it is not considered a ‘spontaneous’ sphere.  If the creator recognizes what they have done, they are still capable of changing the sphere’s settings.

The exact methodology by which a creator changes the settings of their sphere vary.  Many simply can change the sphere with their mind.  Ritually created spheres sometimes involve the creation of a magical artifact which can change the settings - and which allows control of the sphere to be taken up by a new individual, should the original creator die or leave.  Otherwise, if a creator dies, the sphere will retain the last settings it held and be unresponsive to attempts to change it.

(There is debate that spontaneously created spheres are simply ones too old for the original creator to be known where the creator has died).

Once again, though, this leads neatly to our next question.