1. https://www.kiwix.org/en/ https://www.kiwix.org/en/ : see content at https://library.kiwix.org/ https://library.kiwix.org/ 2. Info manuals, info browser 3.

1. https://www.kiwix.org/en/ https://www.kiwix.org/en/ : see content at https://library.kiwix.org/ https://library.kiwix.org/ 2. Info manuals, info browser 3. Self-documenting programs & environments (e.g., emacs but also Python with docstrings etc) 4. Look at the source code on your machine