Connecting

You can only connect two devices if they do have a “protocol” in common.
A simple one to connect your PC to the audio is using a Y-cable (3.5 mm stereo headphone jack to 2 x RCA).

This is a dirt cheap way to get started but don’t expect much sound quality wise.

 

Desktops with a discrete sound card can be connected using the line out.
DAC's and a lot of receivers have a digital in (SPDIF). This might be electrical (coax) or optical but a lot of PCs don’t have a SPDIF out.
In this case a USB to SPDIF converter is an option.

 

Today there is a plethora  of USB DAC's
The new generation supports sample rates up to 24 bit/192 kHz or even up to 384 kHz.

DoP (DSD over USB) is becoming popular.

Today USB DAC's uses asynchronous synchronization.

 

They have USB in and analog out.  

As there is no PC without USB and almost all amplifiers have analog in, this is the ideal bridge between the PC and the audio.

 

Often the DAC has all the digital inputs: USB, SPDIF, Toslink, AES/EBU

Combine this with a volume control, a headphone out and you have pre-amp with digital inputs. This has become common today.

 

More details on how to connect the PC to the audio can be found here.