I know the most about Roman Britain, so that’s where this answer is going to be largely based. Although the narrative for a long time is that the end of Roman Britain was apocalyptic in nature - and this is where the idea of the so-called "Dark Ages" across Europe comes from - this is something that’s been challenged in the last century, mostly through more archaeological information that adds to (and in some cases contradicts) traditional narratives built around the few writers like Gildas.

To be clear from the start, no one has a definitive answer to why the end of Roman Britain seems to be in the early 400s, well before the rest of the Western Empire “fell”. It’s traditionally dated to 410, but the dating (and legitimacy) of the sources that lead to this is tricky. The historian Zosimus, writing in Greek, claims that at some point around then the Britons threw out Roman magistrates and decided to govern themselves - and if this coincides with the sack of Rome and that’s all very nice and tidy. That is, of course, assuming that he was in fact talking about the Britons… and not the citizens of Bruttium in Italy, or, has been suggested, Raetia. The most solid evidence we have - that’s not other extremely vague literary references - is the end of mass importation of coinage to pay the soldiers stationed in Britain sometime around 402. It’s suggested that the shadow-emperor Stilicho who was in charge as the real emperor was about fifteen or sixteen, stripped Britain of many of its troops to fight wars elsewhere in the Empire and they just never came back. We know that many forts continued to be occupied - although for e.g. the granary at Birdoswald became a feasting hall - so it’s possible not all of them remained on the continent or actually went to begin with, but the question remains open.

Although our historical view has mostly been built on the back of Gildas - who tells us about violence, about arriving saxons, and helps kick off some of the King Arthur stuff - it’s important to remember that a lot of the historical detail he gives is outright contradicted by our knowledge from other sources, and that he is not attempting to write a history. Instead, he’s writing a sermon to inform people about how every awful thing that is happening right now is because, frankly, they suck. He doesn’t frame it as explicitly apocalyptic - his model is more clearly the Old Testament and God's various judgements upon Israel - but whatever is happening is clearly very distressing to him.

When we look at both Gildas and St Patrick - the two main sources of evidence for this period - we see fairly well educated and at least semi-Romanised latin-speaking elites who have some familiarity with classical texts. Clearly there were many families who maintained a strong Roman latin-speaking identity for some time. The dating arguments for both St Patrick and Gildas are complicated and I’m not going to go into it except to put Patrick somewhere in the mid to late 400s and Gildas as writing sometime between the 480s and 530s (maybe 550s at a pinch). Either way, they seem to have been from British families and had a classical education some time well after the ‘fall’ of Britain as a Roman province.

The current archaeological evidence suggests that for most people - especially in the countryside - life did not change that much or that quickly. We’ve little to no evidence of mass violence. A lot of cities had already seen their heyday and decline as local romanised elites retreated to their own villas, and may have been converted into centres for the conversion of taxes paid in objects to tradeable goods. The villas became the nucleus of small economic groups before eventually being abandoned. Any soldiers remaining may have become the core of a new elite as they could offer protection from e.g. Irish pirates, but we simply don’t have any evidence to say for certain.

The very gentle collapse we see into “unromanised” life is somewhat different from that on the continent. There’s a good chapter written by Higham in The Anglo Saxon World on this, but to summarise: the province of Britannia may not have maintained much if any of the Roman social structures because it had never been thoroughly ‘Romanised’ to begin with, unlike Gaul where new elites could simply take over the existing Roman administrative system. Britain didn’t really produce any big figures on the stage of the Roman empire; its indigenous elites just straight up didn’t really seem to succeed outside of the province (I think there's Pelagius? that's it). Likewise, Britain is notable for its stunning lack of Roman inscriptions compared to other provinces. Although Patrick and Gildas clearly had a classical education, spoke and wrote Latin and had cultural ties to Rome, this just doesn't seem to be true for the majority of the population.

In terms of stone and lead stripping, the issue is not that people did not remember how to mine lead or dress stone, it was that doing so no longer made sense for them (and also - if you have a source of freely available dressed stone in the form of abandoned and useless buildings, why not use it? that’s definitely the later British take on Roman ruins).

The early medieval period in Britain does seem to be characterised by use of wood. However, is likely that wood was very accessible and had some specific cultural meanings to the people who were using it - and may have represented a return to indigenous forms of construction. Even later halls seem to be built largely in wood, when stone is both available and potentially affordable, although cathedrals and minsters begin to be made of stone. This may make more sense if you think about that as a development coming with people from the continent, and especially from Rome, who are outside of the then-dominant culture.

We do see a drop in the usage of lead, but I would argue that it’s not because people do not know how to obtain it (as it’s one of the easiest ores to extract), but because it’s - again - no longer useful. One of its primary uses was in cisterns and pipes, something that with the death of bath house culture in Britain stopped being useful technology. It was picked up again in the high middle ages when it became useful, e.g. for creating lead fonts for churches, for church roofing, or for cheap tourist objects like pilgrim badges.

In short, people are happy to abandon things which are no longer useful to them, including technology. The technology they use isn’t simply a climb to “higher” levels of sophistication but is mediated by culture - people are often willing to do things that seem 'less advanced' because it makes sense for them and their environment. After the “fall” (more like a gentle saunter downhill) of the Roman Empire in Britain, certain technologies and projects like the building of new aqueducts simply stopped being useful or culturally interesting.

For an indication of the skill of craftsmen in early medieval Britain I would look at the Staffordshire Hoard, which includes gold wire less than a millimetre thick (!). They were very, very good at the things they were interested in doing and which made sense to their culture to do, and were also able to obtain objects like garnets from as far away as Sri Lanka. Early medieval Britain was neither unsophisticated or isolated, it just had very different cultural priorities and levels of civic organisation.

In conclusion in Britain: possible revolution (unclear), very little to no widespread violence, life probably didn’t change very much for most people for a long time and when it did it happened fairly slowly, people did ‘lose’ some technologies but mostly because they stopped making any cultural or economic sense for them, apocalypse rating 2/10.

All of this said, I do have some vague memories - possibly from Peter Brown’s “Through the Eye of a Needle: Wealth, the Fall of Rome, and the Making of Christianity in the West”? - of some continental Christian writers making explicit connections between the end of the Western Roman Empire, the book of revelations and the end of days, so that may be worth looking into for more detail on how some (possibly very excitable) people took the end of the western empire. I'm unfortunately not very well versed in those sources or the post-Roman period on the continent, so couldn't track it down any further without proper research.