It's a optic to electronic device that is embedded in the cable, which is powered by electricity (but I think the tech was improved, see my last link). It's mentioned here: https://en.wikipedia.org/wiki/Submarine_communications_cable... with more detail here https://hackaday.com/2023/08/08/under-the-sea-optical-repeat... and pictures here: https://hackaday.com/2023/08/08/under-the-sea-optical-repeat... (IIUC those are inside of the ship laying or repairing the fiber,a nd they normally live on the ocean floor) and tons of photos of the process of laying cable: https://www.businessinsider.com/google-facebook-giant-unders...

However I think there are also fully passive repeaters- https://en.wikipedia.org/wiki/Optical_amplifier