The precise method depends on the file system in use. In general, it finds the file beginning point in a directory that starts at a known location. The directory will point to a (linked) list of blocks that make up the file. To find specific information in a file, some searching in the list of constituent blocks may be required.
Some simpler operating systems go to the specific track, head, and sector of disk specified by the block you tell the operating system you want to retrieve.
Some simpler operating systems go to the specific track, head, and sector of disk specified by the block you tell the operating system you want to retrieve.