Secondary goddes are an expression of the effect of belief acting on the Primary Goddes. Each Secondary godde is a local pocket of the overall plane -- quite small in size compared to the plane as a whole being exercised over the whole universe. Essentially, the Primary goddes are able to contain secondary (and tertiary) goddes. For all examples to follow, consider the intention and personality of the secondary godde to be in practice the same as the consolidated beliefs their society has about how they should feel and behave in broad strokes.
Formation and Powers
As sentient races began to form into societies, they organically developed spiritual beliefs over time. Many of these beliefs became established narratives that were passed down over many generations. As the community came to view these as unquestionably true, this faith acted upon the planes themselves. Scholars are uncertain whether the planes were split into distinct pockets, or if the godde is merely a fuzzy representation that forms an impression when acting, and fading when not witnessed directly.
Regardless, these are beings that are primarily defined by humanity, but restricted by the physical laws they still must obey / facilitate. While the Primary goddes--which are synonymous with the planes themselves--are beholden to perform exactly what each natural phenomenon demands, Secondary goddes are capable of applying judgement to a situation. Primary goddes are aware of every single reaction of the Material plane under their purview, in that exact instance of time without accounting for the past or future. Due to their smaller nature, Secondary goddes are able to perceive time and other entities in a broad sense.
Predominantly, these goddes are distant from the actions of societies, but they have an effect both on divine magic and history. Their perspective is still large and unwieldy to the mind of a mortal, as their awareness extends to their area of influence -- both in theme (e.g. godde of the sea) and geographic location (e.g. near the culture that believes in them). They act slowly, in large trends over time. A single individual who refuses to participate in the religious standards of their culture will not receive divine retribution, but an entire city becoming corrupted away from the original form of worship will draw attention. The specific forms of their power vary according to the plane and beliefs. For example, a godde of fertility and the harvest might use their influence to bring a great crop, or turn fields of grain into barren soil.
Unfortunately, due to their perspective being so vast and different from their adherents, there is often a large delay (as perceived by mortals) between the trigger and response. Consider a society that believes their sky deity will punish them with storms if they do not provide appropriate offerings on specific days. The godde would notice this drop in the energy gifted to them by their followers, though it is debatable if this always involves a real sentience or merely the reactionary nature of an animal. Their energies would brew in the atmosphere, and finally provide storms. The time scale could be as little as months or as long as centuries. Complicating this is that the godde has a base level of power that is directly increased by investment from their mortals. This means that occasionally a godde might become unpopular so quickly that they are unable to respond, as they were unable to create an effect before their power dwindled.
Unlike Primary goddes, Secondary goddes are accessible to most divine magic practitioners in a mutual exchange. The faith, prayers, and offerings from mortals invest energy into the godde themselves, boosting their planar area in power and influence. Then they are able to use this power in the manner they choose -- often to the benefit of the faithful, as mortals usually believe in reciprocal exchange to some degree (e.g. a generous deity bringing outright magical gifts or a wrathful godde leaving them alone).
This power is also available, to a limited degree, to the various divine mages of significant concentration and skill. The mortal is able to channel the power of the divine in a manner consistent with the planar actions. The specific methods they employ are largely defined by the tenants of their culture. In theory, the mechanics of becoming a divine conduit at a base level only require devout belief and concentration. In practice, the creation of this relationship involves a lot of trial and error, accreting symbolism and methods over time. It can be a messy, free process, or methodical to a stepwise theology, but either way accumulates baggage.
This is not necessarily negative, as the props of special language, physical objects, and performance can make it much easier for a mortal used to physical form to wrap their minds around the abstract nature of the goddes and power. Objects can be invested with different energies that can add power to a spell, and symbols can be invested with their own set of beliefs which collect their own small influence that can strengthen or act as a catalyst to a spell.