A streamer hosts the music and the software and streams it to an endpoint. An endpoint has no software, no storage, no way to play music unless it is sent to it. So the endpoint receives the music from the streamer and sends it to the DAC.
Some Streamers can also act as endpoints.
Endpoints cannot act as streamers.
The reason for the existence of the AirLens is to receive the music from the streamer via Ethernet and convert it to I2S in order to feed the DSD MKII. The DSD MKII will not have an Ethernet connection. With the MKI this job was done with the Optional Bridge 2. The Bridge 2 was designed and built and controlled by another company. PS Audio did not have total control of the Bridge 2 software or hardware. This was an issue. The AirLens is designed to solve these issues for the MKII DAC. I believe it could also be used with the MKI DAC. I might be wrong about that.