How are most audiophiles going from streamers to DACS

Typically optical is of poor quality. SP/DIF (RCA) or AES will have full resolution. Music streamers normally will not have HDMI outputs, those are for home theaters.

A streamer can have a DAC with it. Normally if you talk about a streamer it wound be assumed not to have a DAC. But there are all sorts of combined units. If the unit is a streamer and DAC, then there is likely single digital output for connection to an external DAC and dual RCA analog (and maybe dual XLR) outputs for connection to a preamp.

Do not use optical, that is a rule of thumb. Then whether the USB or COax would need to be tested. Given the inexpensive nature of the two components you could just get two budget interconnects USB and COAX. Since USB re-times the signal, this may sound better. COAX uses the timing of the streamer.