See very high cost in data transfer section - what is the reason for that, any suggestions to get rid of it.

Cost Explorer can help you to find the cause (or atleast the service) causing this. Here is a short example I wrote awhile ago, including links to other helpful resourses.

https://carriagereturn.nl/aws/finops/data/transfer/tag/2022/10/13/tag-data-transfer.html