Ans-1 - 

A kernel is the core component of an operating system that acts as a bridge between the hardware and software. It manages system resources such as memory, CPU, and input/output devices, and provides a layer of abstraction between the hardware and higher-level software components.

It's not just a Linux thing. Windows and macOS have kernels too.

Ans-2 -

Linux, in its nature, is not an operating system; it’s a Kernel. The Kernel is part of the operating system – And the most crucial. For it to be an OS, it is supplied with GNU software and other additions giving us the name GNU/Linux.

On a computer, the latter is afterwards installed as a distribution, such as Ubuntu, elementary OS, Fedora, OpenSUSE, etc. Some believe that we cannot generalize all "Linux-Kerneled" operating systems as GNU/Linux because some Linux distributions include more software than GNU.

Ans-3-
No, Linux is not immune to malware. While it is true that Linux has certain security advantages over other operating systems like Windows, no operating system can claim to be completely immune to malware.