It depends very much on how many cameras are used. All cameras store the raw material on a suitable storage medium such as an SD card, SxS card, P2 card or even SSD, depending on the manufacturer's specifications. In the case of 3 or more cameras, the use of an external SSD hard disk is strongly recommended for the storage of all material from all cameras and thus the double backup of the data on one storage medium. The most common brands would be SanDisk, Seagate and Samsung.
The following services offer data storage via the Internet: