Learn Danish: Hun arbejder som biolog i naturreservatet. - She works as a biologist in the nature reserve.

In Danish, when you talk about someone's profession, nationality, or role, you normally drop the indefinite article (en or et). So, while in English we say 'as a biologist', in Danish it is simply som biolog.

I've seen som used to mean 'who' or 'which'. Why does it mean 'as' here?

Som is a very versatile word. At this level, you probably use it a lot to connect sentences (like 'the man who...', manden som...). But its basic meaning is 'as' or 'like' when stating a role or making a comparison, which is exactly how it is used here in som biolog.

How do I know if a long compound word like naturreservatet takes -en or -et for 'the'?

For compound words, the very last part of the word determines the gender. Here, the last part is reservat, which is an et-word (et reservat). Therefore, the entire compound noun gets the -et ending to mean 'the nature reserve'.