this is me purely being curious: 

it might be just my subjective impression, but I see in my ads across official YouTube stream (I am from Germany), so many ads from netflix for Korean dramas and shows. 

The numbers are hugely outweighing the number of e.g . Japanese, Chinese, Hongkong Dramas  (non-existent, I don't think, there has ever been any HK television dramas on Netflix) , Thai dramas (not sure, if i ever saw some ads on Netflix for them), Taiwanese dramas, Malaysian Dramas and so forth. 

does anyone here is sort of an insider, in the streaming industry, and knows, why and how and based on what criteria netflix choses what to buy from the respective production companies from the various countries?

I know, Untamed was on netflix, a couple of years ago, Rise of the phoenixes was on netflix, i think and there are surely a couple more.

but purely on the number of ads i see on YT, as well as for example, shows being discussed in German TV and streaming magazines? :

south korean dramas are domineering the streaming sites. sure enough, they started earlier than c-drama industry, and as such, they have the advantage of time.

and of course, being a "democracy" , more or less, they have the freedom to make controversial topics in dramas (corruption, rape, toxic hierarchy, criticism of nearly everything and so on), as in contrast to china, where for example , BL dramas have been suddenly blacklisted. 

is it simply, because K-dramas are picking up more controversial topics, that makes them more attractive to the worldwide audience, and as such, netflix is picking them over the dramas of all the other asian countries?