East Frisia and Northern Hesse are SPD strongholds because they are very industrial and very Protestant.* Despite stereotypes they have not actually been rural backwaters for a long time; longer than anyone posting on this forum has been alive and the only North/Western European countries where they would not be fertile ground for the primary social democratic party of record are those in which that party has collapsed into irrelevance or never had much to start with.
*A couple of additional cultural matters add further to SPD strength in the former, much in the way that similar issues add extra percentages to CSU totals in Bavaria and so on.
That makes sense. Obviously in Germany tough to compare to US or Canada as even so called rural areas are far more densely populated than in those two countries thus why right wing dominance not as automatic. However UK is similar here and while its true areas like this were mostly Labour strongholds in UK until very recently, many similar areas Boris Johnson was able to win. Correct me if I am wrong but they seem comparable to Copeland, Northwest Durham, Workington, Sedgefield which are all semi-rural and all Labour strongholds until very recently. Mind you Scholz is a lot more moderate than Corbyn so its quite possible those areas in UK would have stayed Labour if they had someone similar to Scholz. For US comparisons, a good example is Iron Range which will Democrats still have some residual strength, its not the Democrat stronghold it once was. Only reason Minnesota has not moved right is due to Democrat gains in Minneapolis-St. Paul area easily cancel those out. Asides from much lower population density, Canadian equivalents perhaps Northern Ontario, Cape Breton Island, and North Vancouver Island which did vote for Liberals or NDP, but were much closer than historical and wouldn't be surprised if Tories flip those areas whenever they return to power.
Also could fact all parties close to centre and Germany lacks the ideological polarization you see in English speaking world also play a role. Left/right polarization seems much weaker in Germany than English speaking countries so that might help avoid the rural/urban polarization you have there. I know in both US and Canada, Merkel and Laschet would be called RINOs in US or Liberal lite in Canada. Certainly right in Canada and the US unlike Germany expects far more ideological purity and thus left being more dogmatic and turn off in rural areas could be partly a backlash to that. Likewise on left, I believe a lot of the purists tend to be younger generations who are still very idealistic and not older members who are more pragmatic. And SPD did quite well amongst seniors, quite a contrast with Labour Party which skews very much younger like Greens do. To be fair in Canada Liberals do well amongst older voters while its NDP who skews younger, still both are very much big city parties and much weaker in rural areas than SPD, but not quite as urban centric as Greens are.
Not the Cumbia ones. Labour's strength there came from the manufacturing towns along the coast (Workington, Whitehaven, Maryport etc.). The rural areas of the Lake District as well as the smaller towns (e.g. St Bees) have long been Tory voting, but until Brexit the Labour areas could comfortably outvote them. Similarly in a seat like Bassetlaw the rural east of the seat always voted like neighbouring rural Lincolnshire (i.e. very Tory) but until recently it could be outvoted by the working class towns, the largest being Worksop and East Retford.
Durham does have more of a traditional 'rural' Labour vote though this was down to the very deprived pit-villages, not pretty farming/hill country. These areas were already shifting Tory pre-Brexit though as the demographics that made them Labour; working class, collectivist mindset was literally dying off and being replaced by more typical village demographics (i.e. Tory inclined). Bishop Auckland for example swung Tory in 2015 even though Labour did pretty well in working class northern areas that year.
