linux kernel not seen from android filesystem?
The linux kernel for an android device will be built separately from the vanilla AOSP.
Of course, one can configure to include the build of linux kernel inside the Android build process.

The kernel will be compiled and compressed into an image along with the ramdisk image. This is essentially what constitutes the boot.img which will be placed in the boot partition.

BTW there are several partitions divided inside whatever storage is used for the target android device.

While the linux kernel resides in the boot partition inside the boot.img, the android system will be stored in system.img which will be located in the system partition.

For a typical android build, the root account is inaccessible to the user(android system). Thus, a typical android user will not be able to check out the root directory.