Simplified Reality and the Unseen: Because reality is (and only is) what sentient beings believe it to be, aspects of reality that no one knows about, simply don’t exist. This includes all microbes and microscopic flora and fauna; molecules, atoms and subatomic particles; cells, organelles, proteins, DNA - none of it exists, but the macroscopic beings and objects of the world exist nonetheless. Because these things don’t exist, things like cell theory and germ theory are but one of multiple mutually exclusive faiths, each of which are true to those who believe them. For example, one who believes that diseases spread because of divine smiting, can smite their enemies with disease; those who believe that diseases spread by physical contact can halt it by isolating the affected. Likewise, things too far away, such as the stars, are simplified into simply being lights - and one could thus pluck the wispy light from out of the sky and hold it in their palm - and aren’t actually celestial bodies - unless you believe them to be, in which case that faith lets you visit those stars.
The Past: The past only exists insofar as it determines the present (including the memories people currently have, about the past). Recent past events are very clear, as its consequences can be felt in many places; hence the recent past seems very real. However, the further into the past you look, the less it can be seen in the present, since more recent events occlude those of the more distant past. Records disintegrate, memories fade or are changed, artifacts fall into disrepair or are forgotten and thus cease to exist, fossils lie buried under rock, and are forgotten, and disappear. The far past, and the creation of the world, is shrouded in mystery; different peoples have different faiths about the world’s creation myth, none of which are actually more true than any other, but the particular creation myth one subscribes to allows for certain types of magic; so a faith that the world was dreamed into being, makes it theoretically possible that a powerful mage with that faith can dream up their own pocket realities; while a faith that the world was designed by a pantheon of gods allows for worship and prayer to those deities, which can manifest as magic. All these different faiths will explain many things about the world, such as how life came to be, how sentience came to be, how the moon came to be, etc.