The main Linux kernel versions comes from the Linux Torvald who is author of Linux kernel, from his git at https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/ , we can clone this kernel as,
$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
First 5 lines from the top level Makefile in Linux kernel contains information/variables about the Linux kernel versions.
VERSION = 4
PATCHLEVEL = 9
SUBLEVEL = 45
NAME = Roaring Lionus
In Above, first number “VERSION” denotes the kernel version, “PATCHLEVEL” denotes major revision, “SUBLEVEL” denotes minor revision of Linux kernel.
All the latest changes happened in most recent kernel version can be found at https://kernelnewbies.org/LinuxChanges whereas if we want to see what are the changes in 4.9 kernel as compared to 4.8 kernel can be seen at https://kernelnewbies.org/Linux_4.9