Lots of 10 GBit/s Ethernet links are actually optical (e.g. 10GBASE-SR or 10GBASE-LR, see https://en.wikipedia.org/wiki/10_Gigabit_Ethernet) although there is also 10GBASE-T over twisted pair cables with 8P8C ('RJ45') connectors as described by @horta. As far as I know is this quite power hungry compared to the optical variants.
Transfer of data from the CPU (or rather memory) to the Ethernet card happens usually through the PCIe bus in an x86 based computer. PCIe Gen 1 lanes have a useable data transfer rate of 2 Gbit/s second (after the 8/10 bit encoding). With 8 lanes the theoretical maximum is 16 GBit/s (per direction), sufficient to drive a single port of 10 GBit/s Ethernet.
The CPU deposits the data to be transmitted in the RAM and then instructs the network card where to pick up (DMA) and similarly for reception the CPU allocates buffers and informs the network card about it when then typically generates an interrupt when the buffer(s) were filled. Note that the bandwidth to RAM is typically much larger than the one of the PCIe bus.
Today we have PCIe Gen 3 widely available which has useable data rate of roughly 8 GBit/s per lane and direction. A 16 lane slot can theoretically handle 128 GBit/s, sufficient for 100 GBit/s Ethernet (PCIe Gen 4 has been announced officially recently).
So the 'trick' to achieve high throughput inside the PC (without having to go to exorbitant signalling speeds) is to use parallel buses (RAM) or multiple serial lanes (PCIe).
For 100 Gbit/s Ethernet one typically has four links with 25 GBaud signalling speed (100GBASE-SR4, 100GBASE-LR4, 100GBASE-CR4), there are also a standards for cables with ten links (e.g. fibre pairs) of 10 Gbit/s (100GBASE-CR10, 100GBASE-SR10, 100GBASE-CR10). For longer distance links, there are also standards using only a single fibre, either using four wavelengths (100GBASE-CWDM4) or using two polarization modes and QPSK (100GBASE-ZR).
For extremely high link speeds over long haul links (such as the Marea transatlantic cable with 20 Terabit/s per fibre pair) one packs as many transmitters at different wavelengths as possible into the useable wavelength band of the fibres and amplifiers, also known as Dense Wavelength Division Multiplexing (DWDM). Note that such a multiplexer/demultiplexer is typically an optical only device at its core and is fed by multiple lower bandwidth streams which can be processed electronically in parallel.
To achieve 20 TBit/s one then also has use advanced modulation techniques where at each clock cycle multiple amplitudes and phases can be transmitted (I have seen 64QAM in a whitepaper) therefore transmitting multiple bits per clock cycle, similar to the 10GBASE-T standard described by @horta.