\input{file}
The \input
command causes the specified file to be read
and processed, as if its contents had been inserted in the current
file at that point.
If file does not end in ‘.tex’ (e.g., ‘foo’ or ‘foo.bar’), it is first tried with that extension (‘foo.tex’ or ‘foo.bar.tex’). If that is not found, the original file is tried (‘foo’ or ‘foo.bar’).