I have noticed that D&D 5E seems to have skipped over a lot of the original D&D material regarding the creation of magic items. I think it is helpful to know how such items are made in the first place, so that placement within an adventure area makes sense in context. In most cases, creation of any magic item is an expensive and time consuming process, and creation of a permanent magic item requires the very highest level spells. Given the costly nature of creation in D&D, magic items should be fairly rare and highly valued. Here is an excerpt from the AD&D DMG:
"For example, a player character wizard of 15th level desires to make a ring of
spell storing. He or she commissions a platinumsmith to fashion a ring of
the finest quality, and pays 5,000 g.p. for materials and labor. He or she
then casts the enchant an item spell according to the PLAYERS HAND.
BOOK instructions. As DM, you now inform him or her that in order to
contain and accept the spells he or she desires to store in the device, a
scroll bearing the desired spells must be scribed, then a permanency spell
cast upon the scroll, then the scroll must be merged with the ring by some
means (typically a wish spell). As all of that could not be done in time, the
ring would have to be prepared with the enchant an item spell again."
And there was another example for an Illusionist:
"Thus, with a great expense in time, money and preparation, major creation, alter
reality and true sight spells, and an unflawed gem worth not less than
10,000 g.p., an illusionist might be able to create a gem of seeing."