Where BIND9 saves its cache?

You seem to be asking two different questions.

For the first, the cache is held only in RAM. However you can view it with rndc dumpdb --cache which will be saved (on Debian and Ubuntu) by default to /var/cache/bind/.

For the second, I think that perhaps you don't know what a view is with regard to bind.

A view is an alternate zone presented to a subset of clients. For example, if you have a private and public network you might create a public only view of your zone for external clients while internal clients will have a view for public and private networks. You can only flush a named view that is configured in named.conf (or any included files).

A view is not the same thing as a zone, and you can't individually flush zones.

For example in named.conf:

options {
    dump-file       "/usr/local/etc/namedb/working/named_dump.db";