How does Linux, the kernel, mount filesystems? What actually does this?

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http://unix.stackexchange.com – My question concerns the Linux source, and if it actually contains the code necessary, natively to mount, parse, and use a filesystem, or if that's added to the source. If the Linux source contains code to parse and access/organize files through a file system and such, is it safe to say that the filesystem is controlled by a device driver coded in to Linux, or is this procedure differ based on things like distro, releases, etc. How does Linux access files using a filesystem on other storage devices, i.e., how do userspace programs access proc, ext3, ext4, etc., and are ext3, ext4 considered (HowTos)