How are the audio input and output of a transceiver connected in a station configured to operate using FT8?
Why FT8 is a sound card digital mode: the WSJT-X style software on the computer generates the transmit audio tones and decodes received audio. So the transceiver's audio output (received audio) goes to the computer's audio input, and the computer's audio output feeds the transceiver's mic or line input, usually through a USB interface box or a radio's built-in USB codec. The computer also needs a PTT or CAT control line to key the transmitter, and accurate time sync since FT8 uses 15 second transmit slots.
Watch out The terminal node controller answer describes older packet radio, where a hardware TNC did the modulation and demodulation; with FT8 the computer's sound card does that work, and there is no such thing as a standalone FT8 conversion box or website.
FT8 = sound card mode. Audio in, audio out, computer does the rest. No TNC, no black box.