Hello Aniket
HDLC is an encapsulation method that provides layer 2 functionality almost exclusively for point to point connections. Point to multipoint connections are also possible but are rarely used. Ethernet on the other hand is a layer 2 technology based on a multiple access philosophy. These are two very different and incompatible technologies, both of which operate at layer 2, so HDLC cannot operate on an Ethernet interface.
Do you have an example of what you have seen concerning HDLC and Ethernet? Maybe that will help to clarify the confusion. If you do, share it with us so we can continue the conversation.
I hope this has been helpful!
Laz