In my experience there is an implicit understanding that Arabic is at least as close to perfect as it gets. The basic reasoning is that the Quran is perfect and written in Arabic, therefore Arabic is perfect. Nobody says this explicitly so the logic doesn't have to be airtight, but native Arabic speakers in Arabic contexts do tend to behave as if this is the case.
Native Arabic speakers don't speak anything resembling Quranic Arabic though. And modern written Arabic is also very different from Quranic Arabic, to the point native speakers need training to understand the Quran.
I think what you mean is that they believe the Quran itself is literary perfection and sent by God through the last prophet, and that is indeed the belief.
But Arabs neither speak nor write like the Quran, and to the best of my knowledge they are well aware that their written and spoken languages have evolved through the centuries (as evidenced by the many Arabic dialects).
To some extend. Most Arabs would understaff 95% of the words and sentences in the Quran with any special training. Some vocabulary is no longer used and you might need to look those words up. What you often need training on however is called Tafser which is about comprehension. To correctly interpreting what those phrases and analogies imply or trying to communicate with the correct context and be able to correlate different parts of the Quran together. It’s like the difference between an average layman being able to “read” a quantum mechanics book, and actually comprehend it.
However, as mentioned most Arabs also believe that Quranic Arabic is that perfection. As a result, you don’t need to “improve” Arabic, individual people need to learn more Quranic Arabic that’s all. You would frequently see religious figures speaking in Al-Fusha to sound closer to Quranic Arabic. Not quite speaking in the convoluted nonsensical way of the Quran, but close enough to trick people’s ear into it. K-12 education teaches “Arabic” classes to every kid where they learn the “proper” Arabic often using the Quran and poetry from around the same time as the Quran as the reading and study material.
