Inventory of program development utilities functions,libraries,etc. of a linux system

Consider the following example:

You get a linux version, lets call it "X Linux" running on some processor, and you are given the task to write an application which includes a driver, other kernel code, and user space code, for this specific linux version.

The application you will develop will contain about 50% "driver and the other kernel code" and about 50 % "user space" code.

Then you wouldn't want to re-write functions that are already available somewhere in the Linux system, right? Indeed, you only want to write functions that are not availble in this particular linux system.

The question is:

How would you go about to look through the Linux system to see and list all the available: kernel functions, system calls, user space functions libraries, other available code,

that one can make use of in kernel and user space code? Actually, one might not even now that one want some function until he sees that it is available.

It could be compared to a task for carpenter which is given a toolbox and a storage of build material, then he can visually look through all the tools in the toolbox and also all the available material in his storage, and pick and choose what he needs.

But how would a programmer do this "pick and choose" inventory of available "material" (functions and libraries in kernel and user space)?

BR!