Translation:Those bridges which are between the hills are big.
Same remark: some sentences with the similar construction (azok az) are translated with "the" and other with "those" sometimes "the" is accepted, sometimes rejected. I fully agree with Arcaeca that this course is currently full of inconsistencies. This may explain why it takes such a long time to correct them. Let's be patient.
That would be
Azok a nagy hidak a hegyek között vannak.
The difference here is that the statement above is more a differentiator. It implies there are more bridges that we could be talking about, but the ones between the mountains are the ones we are describing as big. Your statement implies that those big bridges are the only ones we could be talking about. The use of "amelyek" is a big deal here.
Your understanding is absolutely correct. The whole exercise here is built around a nuance meaning to emphasise 'that one single specific' versus 'some other unspoken one that is different' and could be understood from the context. As a native speaker I'd never use this structure. Very arbitrary and subjective.
This is an incorrect translation.
The english says: "the bridges between the hills are big" The Hungarian says: "the big bridges are the ones between the hills."
These two sentences do not mean the same thing! The english describes the bridges between the hill, whereas the Hungarian describes how these bridges differ from other ones.