I suppose what I'm asking is if founder effects can inferred in, for a lack of a better term, "normal" European populations. I.E. not unique linguistic regions like Finns or island populations such as the Sardinians. I'm talking about populations like Mainland Italians, Scandinavians, and Greeks. As you noted Italians have a lot of genetic structure due their geographic and cultural history, but from what I understand most ancestry calculators can still find a general "Italian cluster". I'm less interested about clusters involving small regions that I am in how large clusters like the general Italian cluster, general all encompassing Scandinavian cluster, or the Northwest European cluster formed. In the case of the Northwest Euro cluster, do geneticists infer that Northwest Euro descendant peoples descend from shared founder effects, maybe not as severe as the Finns, but comparable in size. For example if this idea of mine is right, I would be imagining that the lBK culture and Michelsberg culture, from which most of the Farmer dna in North/Central Europe comes from, originated from one or two founder effects of just a few thousand individuals each, just a little larger in size compared to the founding population of the Finns that would grow to fill the area and split into different tribes/clans that would mostly marry amongst themselves. And that similar founder effects could perhaps have been the origin of the Corded Ware/Central Euro Bell Beaker cultures, with that being the explanation for why a Northwest Euro Cluster can be detected, since Northwest Europeans would derive 90 plus percent of their ancestry from the same 5 or so founder effects? Of course I could be completely wrong and there was more mixing between tribes, clans, rural areas, and chiefdoms in pre-modern Europe than I am imagining or that populations couldn't have grown as fast as I am speculating.
Sorry, but imo you're wrong.
For one thing, I'm leaning towards the idea that Corded Ware and the Beakers, for example, formed as a mixture of people from the steppe with Copper Age farmers from cultures like Globular Amphora and Cucuteni-Tripolye, with minimal input from farming cultures further west and south.
One group of Beakers, from my interpretation of the relevant papers, especially those by Casey, moved rather quickly from North/Central Europe to Britain. The resulting inhabitants were at most about 10% local farmer partly because there had been a major climate change caused population crash in Britain shortly before the arrival of the Beaker people.
The Celtic speaking population was then invaded by people from the areas of the Netherlands, northern Germany, etc., the "Anglo-Saxons". These people were heavily Corded Ware descended I would think, with perhaps some Beaker, and with perhaps local admixture, yet although Northern European like the Celtic speakers of Britain, they can be distinguished with no difficulty from the prior inhabitants of Britain to the extent that the admixture can be measured rather precisely going east to west, with the majority percentage being about 1/3 if I recall correctly.
Next we have the "Northmen", both as settlers and Vikings, to the extent that almost a majority of England was called the "Danelaw". Whether they were all that different from the Anglo-Saxons I don't know, as I don't specialize in British genetic history.
What I do know is that a recent paper indicated that there was a "southern" shift in the English population since that time, with the trail leading to France. A similar thing happened to the Scandinavian population, and doubtless to Germans, as can be seen in the difference between Southern Germans/Austrians and Northern Germans, for example.
It was only with the Industrial Revolution, imo, that the regional differences in Britain were somewhat blurred, creating more of a distinct "cluster". At the same time, any decent PCA will show you that people from eastern England and people from the Netherlands are very close genetically, as are Yorkshiremen to Danes.
All countries are a mixture of different strains, more or less amalgamated depending on the location and history of an area.
Italy is far less "amalgamated" than any northern European country. Yes, it's true that you can still recognize the cluster formed by Italians, largely because, imo, there is a large Neolithic farmer cluster which covers the entire mainland of Italy and also because the later migrations to Italian affected the whole mainland, although with the differing intensity which creates the long genetic space held by Italians.
That difference between North and South, with almost a break in the cline south of Rome, is because of the differing effect of the migrations to the north and south of that line. The migration from the southeast starting in the Bronze Age hit southern Italy first and more intensely, although that ancestry definitely diffused northward with significant impact. The Italic and Celtic migrations moved into Italy from the north and diffused southward, with no Celtic influence of any importance south of Rome to my knowledge. As a result, after all the northern and central Italian regions, I am closer autosomally to northern Balkan populations like Albanians and Bulgarians and Romanians, and then to Spanish populations than I am to far southern Italians.
That has all started to change, of course, with the migration from south to north starting in the 1960s, a change to which I have contributed, as I married someone of Neapolitan and Calabrian descent.
I would suggest that you zoom out when looking at autosomal differences in Europe and indeed the world. Everything depends on the lens you use. If you look at a PCA of the world's populations, all Europeans merge into one cluster. The differences at that scale are inconsequential.