Your home fiber link may be capable of 1Gbps. Your ISP may be artificially limiting this but the equipment is capable of this. The equipment used for the subsea cables are usually capable of 100Gbps. This is because they use better electronics, lasers and detectors. But then they do not just have one of these feeding each cable, they have a number of these transceivers at different wavelengths and use a prism to combine the light in one end and split it up at the other end. So a single fiber strand can connect several of these 100Gbps links across the ocean. They then take lots of these fiber strands and bundle them together in one cable.

For comparison a video stream is typically around 10Mbps. Your home Internet is technically capable of 100 video streams at once. The high speed links used by ISPs are capable of 10 thousand video streams. When you bundle them together with a prism you can get maybe 250 thousand video streams through the single strand. A single cable is then capable of a few million video streams. And there are around 500 of these understea cables so the total capacity of them is over a billion video streams.

You are however right that even that is still not enough and companies like Google (owner of YouTube), Netflix, Amazon, etc. have problems with the slow bandwidth of undersea cables. So they take advantage of the fact that a lot of people see the same videos and will copy the videos to different datacenters all over the world and then people will stream from their local datacenter.