WHAT IS THE PURPOSE OF THE PHY LAYER?

Physical channels are the ones over which information is transmitted in air interface as bits and symbols. From the protocols perspective, Physical layer is Layer 1 protocol.

You can see from the visualization below, Physical layer is performing coding, modulation, antenna and resource selection.

Support of Physical Layer to Channels:

Take a look at the visualization above. In terms of channels, looking from bottom up approach, the physical layer provides transport channels to the Layer 2, (i.e., MAC layer). These transport channels differ in their characteristics as how data is transmitted, and they are mapped to different logical channels provided by the MAC layer. Logical channels describe which type of data is conveyed and so on as evident from the visualization. Therefore, we can say that Physical layer supports not only upper layers, it also support Transport and Logical Channels.