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A large document requires a lot of input. Rather than putting the whole input in a single large file, it's more efficient to split it into several smaller ones. Regardless of how many separate files you use, there is one that is the root file; it is the one whose name you type when you run LaTeX.
\include{file}
The \include command is used in conjunction with the \includeonly
command for selective inclusion of files. The file argument is the
first name of a file, denoting file.tex. If file is one the file
names in the file list of the \includeonly command or if there is no
\includeonly command, the \include command is equivalent to
\clearpage \input{file} \clearpage
except that if the file
file.tex does not exist, then a warning message rather than an error
is produced. If the file is not in the file list, the \include
command is equivalent to \clearpage.
The \include command may not appear in the preamble or in a file read
by another \include command.
\includeonly{file_list}
The \includeonly command controls which files will be read in by an
\include command. file_list should be a comma-separated list of
filenames. Each filename must match exactly a filename specified in a
\include command. This command can only appear in the preamble.
\input{file}
The \input command causes the indicated file to be read and
processed, exactly as if its contents had been inserted in the current
file at that point. The file name may be a complete file name with
extension or just a first name, in which case the file file.tex is
used.