rootfs
This is the filesystem created during the operating system installation. It's then mounted at '/' (root) during the boot process.
During the boot process, the '/' directory is mounted with initramfs file system. This then swapped with the real root filesystem (rootfs) that was created with the operating system installation.
This switching is done by commands such as pivot_root. It's the same logic that's also used by docker containers to switch from the host filesystem to the image's rootfs.
/bin - essential system binaries
/etc - configuration files
/lib - shared libraries
/home - user directories (optional, depending on the setup)
/dev - device files
/proc - virtual file system for system information
/sys - another virtual file system for kernel-related information
Kernel vs userspace applications
These filesystems aren't used by the kernel itself. It has all tools necessary for userspace applications to interact with the kernel.