Sure: every cable company has a "headend," which is essentially a master-control facility where incoming signals are received, processed and distributed to customers.
The national and regional networks generally arrive by satellite, while local channels can arrive in a number of ways. In the past, they were picked up directly over the air or brought in by private microwave networks; today, there are often fiber connections directly from the station to major cable systems.
Cable companies insert local advertising at the headend, usually played off digital video servers. The headend is also where the local content is inserted for services like TV Guide Channel and The Weather Channel. It's also where local cable channels are inserted, if there are local channels on the system. And it's where the servers live for on-demand channels and other interactive services ("Start Over," for instance.)
Back in the day, there used to be lots of individual local headends, but with the availability of cheap fiber those headends have largely been consolidated into larger regional headends. (If I still lived in Waltham, for instance, I think my Comcast headend would be way out in Stow, rather than the local TCI headend that was the bane of my existence in the 90s.)