This is one of those questions where there isn't really a great answer. In some cases, it is plain that certain names on lists of demons or spirits or featured in incantations or operations were borrowed or derived from existing traditions; others are of unknown origin. For example, we sometimes see faeries included in grimoire material as is the case in The Book of Oberon: A Sourcebook of Elizabethan Magic. u/itsallfolklore might be able to talk more about how the names of spirits can be a confusing thing, and how the desire to catalog them can lead to misunderstandings.

Other names may represent a corruption of the name of a known deity or entity - since manuscripts copied by hand, possibly from oral sources, and translations or transliterations from Hebrew, Egyptian, etc. into Greek or English are likely to introduce a great many errors or variations of spelling, which can "build up" over time to make an existing name unrecognizable. "Balam" from the Lesser Key of Solomon, for example, might originally have been a reference to the Biblical magician Balaam, which is cognate with or possibly derived from the Semitic word Baal for "lord" - which was often applied to deities, who were later degenerated to demons, but which also survives in the form of ba'al shem ("Lord of the Names"), a Hebrew term for a magician whose power derived from knowing the secret names of God.

In part, the relative lack of individual names of spirits or demons in the Bible is representative of a gradual change in religious and occult traditions. George A. Barton writes in "The Origin of the Names of Angels and Demons in the Extra-Canonical Apocalyptic Literature to 100 A.D.":

In the earlier time the various angels and demons in which the Hebrews believed were not sufficiently personal to bear individual names. Apart from Satan, Azazel, Rahab, Leviathan, and, possibly, Lilith, we find only names for classes of beings. A great change is traceable in the literature of the second century B.C. and the centuries which followed. Proper names were then bestowed upon many spirits both good and bad. Two of these names, Gabriel and Michael, occur in the Book of Daniel (8 16 10 13. 21), but the apocryphal literature affords a considerable number.

So even before you have lists of spirits for conjuring or magical operations, it's apparent that there were apocryphal texts that were going into greater detail on the names (and sometimes attributes) of various spirits. This is likely the origin of at least some of the "barbarous names" invoked in various works of magic: they were not necessarily created out of whole cloth, but might well be derived from certain traditions outside what would become canonical religious literature.

So for many names, we don't have any kind of firm origin or etymology; the best we can do is track appearances and changes in spelling and/or attributes.

Of course, some magicians or writers on magic probably just made stuff up.

This is more obvious in contemporary literature and magick, where folks like Aleister Crowley are known to have coined new words, names, and phrases for their own purposes. The ancient charm-word "Abracadabra" for example was turned into "Abrahadabra" by Crowley for his own purposes. H. P. Lovecraft famously invented names like Cthulhu, Yog-Sothoth, Nyarlathotep, etc. for his fiction, which magicians like Crowley's secretary Kenneth Grant incorporated into his own magical works and operations. There is no reason to believe that something like this didn't happen at some point in the past, and that the details of the origin of many of these names have been obscured but continued as part of the Western esoteric tradition.

So then, where did the material come from? Did new mythology just sort of appear in the centuries between Jesus and the Renaissance?

Like a lot of apocryphal gospels and other religious or quasi-religious materials, we don't always have a firm idea of why people would sit down and write these things. Some of them were probably motivated by genuine bouts of devout faith, possibly by a vision or a dream - many grimoires and religious works, like the Book of Mormon, are supposed to be "received" texts narrated by a spiritual source or intermediary, which origin provides an excuse for how the author could come to know these things and why there were written down. Others could be works of scholarship, efforts to collate and possibly systemize diverse magical practices - as with the Dictionnaire Infernal (1818). But for most, we simply don't know.

While there's always more to say on the subject, you might be interested in u/AncientHistory's answer to How are demons from the Ars Goetia related to Christianity?

If you're interested in that particular passage, you can get some answers on particular meanings from a footnote in Joe Peterson's online Heptameron:

http://esotericarchives.com/solomon/heptamer.htm

In addition to misunderstandings of Greek passages, other languages with phrases that might be reinterpreted and transformed into magical words include Hebrew, Arabic, and - in one Italian source (Secrets of Solomon) - English. There are probably also others that are independent creations, although the number of these continues to shrink as more people look at more texts.

Also of note for further research:

Véronèse, Julien. “God’s Names and Their Uses in the Books of Magic Attributed to King Solomon.” Magic, Ritual & Witchcraft 5, no. 1 (2010): 30–50.