I was under the impression that all modern DAC’s have buffers, that then use the internal clock. Older DAC’s without the buffers used the external clock.
@glennewdick USB audio data is asynchronous, and is fed into a FIFO buffer in the DAC. The release of data from the DAC’s buffer is timed by the DAC’s internal clock. Open a DAC, and its clocks can usually be found on the USB transceiver board.
Data transmitted to the DAC via SPDIF/AES EBU/I2S connections differs in that it is synchronous, containing both audio and clock data from the source. The source holds the master clock, and the receiver (DAC) is the slave. As such, source clock quality is a defining influence of overall audio quality, and expensive streamers put emphasis on clock architecture for this reason. A few upmarket DACs (e.g. Gustard A/X/R-26 and later, Holo Audio May, etc.) have a PLL feature that allows the DAC to slave the source clock to its own.
Popular external DDCs (Digital-to-Digital Converter) virtually all convert USB to a synchronous format (SPDIF/AES EBU/I2S) that makes the DDC the master clock in the chain. Predictably, they also emphasize clock accuracy as a selling point.
