Hardware Haven made a great video on how to make a local self-hosted security system. That should be a good source to get you started :)

The video shows that even security cameras by companies like ADT will spy on you. Apparently 2/3 of US households have outdoor security cameras, and 1/3 have internal security cameras.

Most people probably don’t know how to set up their own secure entirely local security camera setup, so they just hire it done, which almost always means connecting it to a corporate cloud.