The more commonplace an item is, the less we need to decide its origins, but the more unique it is, the more audiences will wonder. Are there people who specialize in creating magic items? If so, these are presumably for sale. Their items are likely higher quality as they are more experienced. They and their store likely have good protection, physical and supernatural. How common is this? Depends on the prevalence of magic; if magic’s rare, these would be correspondingly unusual, probably located in the populous and wealthiest cities. The profession for these people can be named for ease of reference, possibly by the type of magic. By doing this, we can create more highly prized items that characters are thrilled to find or possess.
Some items may originate with the gods, whether for their use, for use by demi-gods (like Cupid’s bow), or for mortals. If it’s for the gods or demi-gods but a mortal possesses it, it’s likely been misplaced or stolen. Why would deities create items for mortals? The mischievous or villainous ones might want to give their followers an edge. Benevolent gods might subsequently try to counter that by granting special items to their own, but they may just wish to reward someone. Or perhaps use of the item benefits the god in some way, such as attracting followers eager for it to be used on them. The recipient is likely a member of their religious order, but needn’t be. Consider the god’s aims.
The item’s physical form has implications that render inventing them easier, but form often has little to do with function. A magic broom may be used as transportation, which has nothing to do with its form and function (to sweep). Rings and wands have little to no impact on how they’re used, but a weapon or tool does. Consider our use for the item and if it makes sense to marry function to form.
Wearable items have the advantage of seldom being left behind by accident. They’re also with us when we suddenly need them. This is why jewelry is so commonly used it’s become a cliché. Jewelry often serves no real purpose, making it ideal for magic properties that have little to do with form. A ring that makes the wearer invisible is more plausible than one which bakes cookies, as the spell applies directly to the wearer. Worn items typically affect a person for as long as activated. Whether magic or not, some weapons can be worn but might not be allowed in certain places, which can pose a problem if characters must leave behind a crucial item. They might just skip going in.
Some items are not expected to travel much if at all, like a tea pot or genie’s “lamp” unless an existing one elsewhere won’t do. Smaller magic items lend themselves to being carried in hand, bags, or pockets; transporting others might not be feasible. This is truer with freestanding items, such as furniture, transportation, or magical doorways. Such items tend to be guarded. What sort of protection does it have?
We should decide who can use each magic item. Knowing the item’s purpose and our story’s need for it guides this decision. If the item accentuates a wizard’s power, like a staff or wand, then it may not work at all for others or only provide simple functions, like casting a light. Generally, a wizard wouldn’t want a non-wizard from gaining much ability from their items.
Can someone without magic talent use it? Without a “magic word” to control some, even they might fail. Some items could be activated or deactivated only by wizards but be functional for others. If there are different types of magic in our world, are people able to use all of them, or only the types for which they have affinity, talent, training, or skill? When inventing a magic system, we will decide on such rules and can apply them to items. Worn items can be lost and used by someone else, while anyone can come across a freestanding item, depending on where it’s located. Consider the likelihood of someone else using it and whether the inventor would’ve inhibited this.