Data is placed in volumes for management. Volume currently only supports local directories. Volume allows many containers to access the same data.
Using docker to manage databases can facilitate multiple database instances running on one host. csphere can also manage data containers very well

I feel that the data is not stored in the docker instance, but in the directory where the host is mounted or using nfs

This mainly depends on your application scenario. In fact, there is nothing suitable or inappropriate.

According to the book, the performance will be better if the data is mapped to the host file system using -v method

Managing Data in Containers is Docker’s official description of data management in containers. I hope it can help you.