The description of tex format expansion
A tex file is a source document prepared by LaTeX, a high-quality typesetting system. It may include graphics, mathematical expressions, symbols and text. tex files are commonly used in computer science, electrical engineering, statistics, economics, and physics for typesetting articles, books and other publications.
TEX files contain plain text unlike the docx and odt files created by the Microsoft Word and LibreOffice Writer WYSIWYG programs, which contain formatted text. The TEX file usually contains markup conventions that define the structure of the document such as a letter, book or article.
Since TEX files contain plain text, they can be opened with any text editor. However, the files are most commonly used by the special apps, which can handle exclusively these types of files.