 example, for 1920 only, Los Angeles County, CA, Enumeration Districts 1-11, 13, 14, 33-37, and 16-32 are located in microfilm publication T625, Fourteenth Census of the United States, 1920, roll 102, while other enumeration districts for that county are located on rolls 103-120.
Note that the soundex indexes and census schedules are on microfilm; you will not be able to search them on-line. Census microfilm is also widely available at public libraries, state archives, and other institutions with large genealogical collections.