Linux – Where would I start looking for documentation on the graphical mode of the Linux console

Things you find in the man pages are generally about programming, and the C API. Non-programming documentation is generally found in the kernel itself.

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/networking/ip-sysctl.txt?id=bb077d600689dbf9305758efed1e16775db1c84c#n843

And those random google sites aren't usually wrong. I'm sure you'll find that many of them are just online versions of the kernel documentation.

First : Credit goes https://stackoverflow.com/questions/15983872/difference-between-user-level-and-kernel-supported-threads

User threads and Kernel threads are exactly the same. (You can see by looking in /proc/ and see that the kernel threads are there too.)

A User thread is one that executes user-space code. But it can call into kernel space at any time. It's still considered a "User" thread, even though it's executing kernel code at elevated security levels.

A Kernel thread is one that only runs kernel code and isn't associated with a user-space process. These are like "UNIX daemons", except they are kernel-only daemons. So you could say that the kernel is a multi-threaded program. For example, there is a kernel thread for swap. This forces all swap issues to get "serialized" into a single stream.

If a user thread needs something, it will call into the kernel, which marks that thread as sleeping. Later, the swap thread finds the data, so it marks the user thread as runnable. Later still, the "user thread" returns from the kernel back to userland as if nothing happened.

In fact, all threads start off in kernel space, because the clone() operation happens in kernel space. (And there's lots of kernel accounting to do before you can 'return' to a new process in user space.)