Magic items are crafted using a number of skills, statistics, rituals, and other abilities in combination. Their creation is highly difficult and they wear out. If you want to make magical gems, for instance, you must be or work with a master jeweler. Low level items can be crafted but they are rarely 'hard' magical items like weapons and armor. The lower the level of dweomer used to craft a magical item the lower its level and the faster it wears out. This principle can be stretched but you can only spread thin power just so thin.
Most item crafters specialize and thus create items in an extremely narrow band of power and function.
Magic items that wear out almost never leave anything of use in their wake. The metal of a magical sword for instance will become brittle and unusable perhaps crumbling into shards. Mages throughout the known world worry that eventually all substances will be thus drained of any potential for magical use at all and the world will be empty of magic. Many commoners secretly pray for that day.
Magic items are usually storied and there is power in knowing the history, maker, name if any, and intent for any given item. There are many stories where the sword of a black-hearted knave refused to strike his foes justly. Likewise of staves that sought out and supported even those without the skill to use them. Magic is mysterious, changing, and interactive.
Most magic items of any power are items requiring attunement. Some items may be attuned when identified only and others by simply coming into contact with them. Like other item rules this issue differs almost per item. In general though it takes continual proper use of an item from sunup to sundown or sundown to sunup, called period acclimation, to attune an item. At some point during this process there must be an extended rest. While an item is attuned properly it often resists attunement from other wielders. See specific rules under attunement.