hat is the proportion of sop during the assessment process?
I don't know the answer to this, because every school is different.
Secondly, my university is one of the best universities in my country(turkey) and it is very difficult to make high grades, so in my sop should i mention the qualities and properties of my university?
I thik you shoud mention the qualities of the university in order to show that it is better than other universities for smeone with your specific interests.
Lastly, how much shoul i give information about my research interest?
In my opinion, it is good to explain a lot about your research interests and your specific plan for the tme you will spend at this school with its esteemed professors.