The -ся ending indicates that it is reflexive present tense - it's not simple present. Reflexive verbs are more difficult to comprehend, and you just have to gain exposure to them and familiarity with them to grasp their meaning.
In English, reflexive verbs usually require a reflexive pronoun: "I see myself in the mirror". When it comes to translating reflexive verbs from other languages into English, the "-self" pronoun is often if not usually dropped, and the sentence is changed so that it doesn't look reflexive at all. The reflexive verb makes sense in the foreign language, but not in English.
Literally translating the exercise as a reflexive English sentence, you'd have to say, "She interests herself in opera", but that's bad idiomatic English, so "herself" is dropped, and the sentence changed to "She is interested in opera". Still, the thing "opera" reflects back onto the subject "she", so in operation the sentence has a reflexive quality - but it's not exactly an English reflexive sentence.
"keen on" is not the same thing as "interested in", probably. You can be interested in something without being keen on it, though being keen on something implies that your are interested in it.
I am interested in the politics of the right wing in the US, but I am the exact opposite of being keen on those politics. I can't stand right-wing policies because I think they're hideous and inhumane - but I am interested in knowing about them.