Many people think that the beer they brew in Germany is the best in the world.
Many people think that German beer is the best in the world.
Both sentences have exactly the same meaning, and given that using "the" is based on the meaning of the sentence, I can't see why native English speakers use it only with first sentence. In other words, why "beer the brew in Germany" is specific and "German beer" is general?
The two sentences have the same overall general concept, but they phrase the concept in somewhat different ways. The use of the definite article is not dependent on the idea conveyed by the sentence as a whole but rather on the specific way a particular noun is used.
In the second sentence, "German beer" refers non-specifically to all beer brewed in Germany, so there is no definite article needed. If "the German beer" was written instead, it would appear that the sentence was referring to one particular German beer.
In the first sentence, "beer" appears by itself, without the adjective "German", so this instance of "beer" could seem to refer to all beer in general. However, the clause "they brew in Germany" specifies that the earlier word "beer" actually refers only to a specific subset of beer. Because the first sentence is specifying a particular group of beer (that brewed in Germany) within the greater range of all beer, the definite article is used.
Jason has correctly pointed out that "German beer" can simply refer to a style of beer. Not all German-style beer is brewed in Germany, and I assume that not all beer brewed in Germany is of the original Germany style. My answer assumes that "German beer" here refers to beer brewed in Germany in order to analyze the grammatical concepts, but please be aware that that assumption is a simplification.
As a proper adjective, "German" can be just as determinative as "the". As a relative clause which is placed after the noun which it modifies, "[that] they brew in Germany" cannot function as a determiner.
Let's ignore the country of origin for a moment:
Many people think that beer is the best thing in the world.
In this example, native speakers would not use "the" in front of "beer". There's nothing that makes this beer any different from any other beer. It's all just beer.
Once we add some further specification, we have reason to also add some determiner:
Many people think that the beer from Germany is the best thing in the world.
Many people think that all beer from Germany is the best thing in the world.
Many people think that some beer from Germany is the best thing in the world.
Many people think that one beer from Germany is the best thing in the world.
That further specification might itself also be a determiner:
Many people think that German beer is the best thing in the world.
Or, it might just be a part of a description that still requires some other determiner:
Many people think that the German beer in this mug is the best thing in the world.
Determinative and specific are related concepts, but they are not identical.
