The Linux kernel is distributed under a general public license, so it is not a public domain software. Concerning the aforementioned, we can say that:

Public domain means that the authors have relinquished the copyright of the code, but the copyright on the software is still maintained by the various authors of the software.

The main implications of the Linux licensing term are that anyone who creates a derivative product can make it their own product.

A software under ageneral public license must be redistributed as a binary-only product.

A software under a general public license does not require any source codes to be distributed along with any binary distributions.

Public domain means that the authors have relinquished the copyright to the software, but the copyright to the Linux code is still maintained by the various authors of the code.