OpenDocument - Wikipedia
https://en.wikipedia.org/wiki/OpenDocument
OpenDocument Text. Filename extensions. .odt .fodt. Internet media type. .odt and .fodt for word processing (text) documents. .ods and .fods for spreadsheets.
ODT File Extension - What is an .odt file and how do I open it?
https://fileinfo.com/extension/odt
ODT files are formatted using the OASIS XML-based OpenDocument standard. This standard is used by OpenOffice, LibreOffice, and other variants, all of which save documents as ODT files.
What Is an ODT File (And How to Open One)
https://www.lifewire.com/odt-file-4135594
An ODT file is an OpenDocument Text Document file. These files are created with and opened by OpenOffice Writer, but some other document editors can also open them.