I2S is the native format of digital audio used internally in a DAC. The data will be in I2S format at the input to the DAC chip itself. I2S includes the following 4 signals: data, bit clock, word clock, and masterclock. A DAC which uses SPDIF input, will have an SPDIF receiver, which interprets (often poorly, and always with added jitter) a single signal, and then creates I2S from that single signal, and then sends the I2S feed to the DAC chip (or FPGA, etc).
The above is for PCM, for DSD, the signals are: data left, data right, bit clock, and masterclock. For some DAC chips (example, ESS), PCM and DSD can be multiplexed on the same I2S inputs to the DAC chip.
It is important to understand, that I2S signaling was developed at first to be used internally in DACs (and CDPs), as the signal is single ended and can degrade over distance. BUT, the I2S signaling used externally by the Sonore Signature Rendu is known as LVDS, this is a balanced version of I2S (so 4 signals running on 4 twisted pairs of wires which are balanced) and does not degrade over normal interconnect distances (a meter or two). An HDMI cable is used because there are good, high bandwidth HDMI cables with the proper impedance and enough twisted pairs. LVDS I2S signaling as used by Sonore (and PS Audio for one example of a compatible DAC) results in much lower jitter than SPDIF interfaces which are compromised by design.