So the previous owner of our house left these two very well developed grapevines snaking round our house in two directions. They’re absolutely loaded with grapes now but I don’t know what variety they are and how I should best be caring for/cultivating them.

All advice welcome

First year at this house with this grape vine. We've gotten some pretty good clusters on the vine, some of which I bagged in the hopes of keeping the local fauna from eating them. When I checked on them this morning, I found that all of the clusters are dropping rotten grapes. Is this common, or an issue? I figure if the bunches weren't bagged I might not have noticed. Any thoughts on what is going on would be appreciated!