Learn Polish: Ci, którzy kolekcjonują komiksy, tworzą wspaniałą społeczność. - Those who collect comic books create a wonderful community.

Ci, którzy is the virile (masculine personal) form. We use it when referring to a general group of people, which implies a mixed group or a group of men. If we were talking specifically about a group of women, or about non-human nouns like animals or objects, we would use the non-virile form te, które.

I know społeczność is the direct object here. Why doesn't it take an accusative ending like -ę?

Społeczność is a feminine noun that ends in a consonant (using the common abstract noun suffix -ość). For feminine nouns ending in a consonant, the accusative form is identical to the nominative. You can still tell it is in the accusative case because the adjective modifying it takes the standard feminine accusative ending: wspaniałą.

Why use tworzą for creating a community instead of a verb like robią?

Tworzyć społeczność is a natural collocation in Polish, similar to the English phrases to form a community or to build a community. The verb robić (to make) is generally too physical or generic for abstract concepts like relationships or communities.

Is komiksy in the nominative or accusative case here?

It is in the accusative case because it acts as the direct object of the verb kolekcjonują (they collect). Because komiks is a masculine inanimate noun, its accusative plural form looks exactly the same as its nominative plural form.