Do users enter commands directly into the kernel of the Linux operating system?

No. In operating systems a user never interacts directly with the kernel.

The kernel is the core of the operating system. It's job is to maintain kernelspace and to facilitate process management, memory management, hardware access control, and interprocess communication. Not to mention provide a framework for device drivers and the hardware abstraction that results.

There's not only no direct line between the user and the kernel, there's absolutely no purpose in a user interacting with it. The kernel only gets "messages" from the user through system calls, which are made through low level libraries like glibc by userspace applications, not by the user.