I cannot say how the whole thing is handled in a lingististically manner.I am neither a linguist nor a Germanist. However, I do not agree with the answer of John, who gives almost every German speaker his own dialect.
I would take a different approach that was valid at least until the 3rd millennium.This approach is historical and partly refers to settlement areas of Germanic tribes. For example, I grew up in the Alemannic language area. It is enormous, stretching eastwards in the west, starting from Alsace to including the Allgäu. In the north, the Alemannic bordered on Hessisch and Franconian, somewhere along south of the Main. The Allgäu and the “Swabian” speaking Bavaria around Augsburg are part of this. In the south, the entire German-speaking Switzerland up to the western part of Austria. A really huge area, once the tribal area of the Alemanns. Within this area there are other sub-dialects in the Alemannic dialect. This can affect individual villages, larger or smaller linguistic microcosms.
How to evaluate such a thing scientifically is one thing.It is true that nothing is static, but everything is in flux. But I’m afraid that’s the way it is with life. If you don’t want to define anything here because of the constant change, then it will also be difficult with language itself. The “English” which unfortunately takes up enormous space is a good example. This is undoubtedly German, many loanwords were also adopted in the Duden. After all, this is the benchmark for German. But when two “Buisiness” types talk about their “org-charts” and the “work flow”, there is hardly any German vocabulary left but German grammar. Even if this is slowly being corrupted by what I consider to be a sloppy approach to language. Dialects are much more conservative and integrate lean words more hesitantly and sparingly. Language is socially bound and is therefore often used politically.
While dialects today appear rather rural, cities are characterized by either High German or Kiezdeutsch.This is a development that can be traced back to immigration.
Young people also like to set themselves off from adults and are quite creative, both within the dialects, the neighbourhood languages or High German.How much of their language is ultimately transferred over time to the written or high language is apparently a matter of general acceptance by a majority of speakers.
In spite of everything, there are still a larger number of distinct dialects, some of which are very popular.For example, Plattdeutsche in the north with numerous local variants, Low German dialects on the Rhine and Ruhr, the famous Saxon, Berliner Deutsch, Hessisch, Fränkisch, Alemannic, Bavarian and some more, but still significantly less than 80 million.
That is certainly not the desired answer to the question.But i am simply concerned with a more conscious approach to language. Our world today has become more open, and that is a good thing. But I believe that globalisation and migration should not mean a death sentence for dialects or even language in general. France, for example, is “fighting” against the islamic state against the isation of French. Whether this is such a good idea, I cannot judge. But the pressure to adapt today is enormous. Ultimately, cultural diversity suffers. On the other hand, however, it makes no sense to maintain something artificially when the majority of those affected no longer invest in it.
Dialects are, in a way, relics from a time when local social group ties were much stronger.This will hardly be maintained, as entire villages and areas are now completely depopulated. With the remaining old people, a local dialect dies out.