Breakdown of Она проходила стажировку в Австралии, ей было интересно.
она
she / it (for a feminine noun)
в
in / to (direction)
быть
to be
ей
to her
Questions & Answers about Она проходила стажировку в Австралии, ей было интересно.
The English translation says she "did" an internship, which sounds like a completed past action. Why do we use the imperfective verb проходила instead of the perfective прошла?
This is a great example of the "general factual" meaning of the imperfective aspect. We are simply stating the fact that this activity took place in her life. The sentence doesn't focus on the result, the exact timeframe, or the successful completion of the internship (which the perfective прошла would emphasize), but merely points out that the experience happened.
Why do we use a form of the verb проходить (to pass / go through) here instead of делать (to do)?
In Russian, you cannot literally "do" an internship. The standard fixed collocation for completing a period of training or an internship is проходить стажировку (imperfective) or пройти стажировку (perfective). You are essentially "going through" the training.
Why does the second half of the sentence say ей было интересно instead of она была интересная?
If you say она была интересная, it means that she was an interesting person! To express that someone found a situation interesting, Russian uses an impersonal construction. The person experiencing the feeling is in the Dative case (ей), paired with the neuter past tense verb (было) and an adverb (интересно). It literally means "to her it was interesting."