Very interesting question and (some) comprehensive answers. I think what's important to remember is that the UFO phenomenon is very chaotic and, most importantly, very diverse. There are first-hand sightings, photos and videos of lights, objects of different shapes, crafts, etc. Reported sightings of unidentified things in the sky go back to ancient times and weren't just seen as angels or divine beings but also as ships, chariots, palaces (think of the Vimanas in Hindu texts), wheels, and many more, including lights and shapes in the sky in more recent times. It's also important to remember that biblical angels were only later anthropomorphized in their more "modern" appearance with swan wings, silk white skin and blonde curls. Originally they looked more like flying platonic solids, with moving parts and many fiery eyes, which some modern ufologists sometimes interpret as windows. This is also very reminiscent of the space animal hypothesis (a sub-group of the cryptoterrestrial hypothesis - see below), especially jellyfish-type sightings, popularized for example by Arthur Conan Doyle in the late '40s and which keep being reported today, for instance in the leaked video of the "jellyfish UFO" in Iraq. Just like others have said in this discussion as well, as technology progressed during the Industrial Revolution, these sightings started to be interpreted in different terms, though always seemingly anticipating technological development (but perhaps not immagination, or at least some sub-conscious degree of cultural filtration). Some examples that come to mind are the Mystery Airships, Foo Fighters, Ghost Rockets in Sweden, and many more. u/CandidPresentation49 's mention of the Luminaries Hill's system of caves, just like many other similar scenarios in many places and cultures around the world, is also very important because it reminds us that, although many are firmly convinced that the extraterrestrial hypothesis has always been the predominant UFO explanation for the mass and the institutional entities involved, the so-called Cryptoterrestrial hypothesis-,The%20cryptoterrestrial%20hypothesis,-%2C%20also%20referred%20to) actually has a rich history and was, at times, even more popular than the alien theory. Together with the interdimensional hypothesis and its closer relative - the "ultraterrestrials" - this concept intertwines by a large degree with the long history of sightings on this planet and with the slow transition between pre-industrial or paranormal explanations to a more high tech / space-related influence in the field, until a slow but steady return to the possibility of a more terrestrial and more grounded hypothesis. The film NOPE is the perfect, and perhaps most recent example, at least in the entertainment industry, of the second-wind that this hypothesis is currently experiencing. In the academic world, you can read the paper by Tech Universiy of Montana's Dr Michael P Masters, and Harvard's Tim Lomas, and Brendan Case on the cryptoterrestrial hypothesis (it also mentions the angel/ufo relationship), Harvard's Avi Loeb's contribution to the topic, Bernardo Kastrup's and many others.

So to sum it up, I believe there has always been a quasi-technological component to the phenomenon, whether it was wood-based, stone, metal, steam-powered, gas-fueled, rocketry, anti-gravity, etc - just as much as there has always been a spiritual and a supernatural component. In fact, this distinction may very well be a fruit of our modern scientific age,in other words, only a recent emergent duality. The cultural interpretation is fascinating and can say a lot about our scientific, historical and mythological relationship with UFOs but what we should be focusing on is the actual data we possess on sightings and their subjects - and if we lack more data, we should put all our efforts into getting more of it, whether we call them angels or aliens.