I2S, S/PDIF, Toslink connections

In searching for a new DAC the input choices are many. There are several ways in which inputs can be connected to a DAC and this can be compounded when an USB/SPDIF converter is inserted between computer and DAC. I wish to learn more regarding the SQ as afforded by these inputs.

Specific questions:
I am of the understanding that coaxial is superior to toslink when connecting CDP to DAC. Is this true?

What is an I2S connection and what are advantages to this option? Uses an HDMI cable i believe.

Thanks in advance for helping with my understanding.