I have googled the first question but no obvious result appeared for beginners like me. So I gather two important question into a post so that other people can find. Please correct me if I misunderstand something.
According to mannages.ubuntu
The kernel-package package use to automate the routine steps required to compile and install a different (custom) kernel.
So Why we need to install kernel-package? ask.ubuntu
There is a project, started and headed by Linus Torvalds, that maintains the source code for a kernel called the Linux kernel. This is what's distributed at kernel.org. Note that this is not a kernel, it's source code — Like a recipe for a cake but not a cake: To make a cake, you have to follow the recipe step by step, mix ingredients, bake; To make a kernel, you have to configure the source code for your computer and compile it.
Thanks for the reference link but for beginners like me, it's contain too much definition. i.e: "upstream kernels", "Source patches"... although explained very detail. I think it's better to use analogy.
– Tan Nguyen Sep 12 '21 at 03:10kernel-packagenot an actual package that is a kernel. These comments read like the Ubuntu version of who's on first. – mchid Jul 16 '22 at 01:36