malloc() is higher level than what the kernel does.

At the lower syscall level you move the BRK address, which is the highest memory address you're allowed to use. By default this is just after the statically initialized memory.

malloc() is just a library that manages this memory for you.

Linux has no idea if you will use the memory you just allocated, usually this happens dynamically; when you access a memory region for the first time, it is allocated in memory for real.