Atom files are called Atom Syndication Format files. The Atom format offers the possibility of platform-independent information exchange. Likewise, RSS can be easily published with Atom changes, for example, new articles and headlines on websites (eg blogs, news sites, etc.).
These XML-based text files can be edited in any text editor, but they should be created or edited using an RSS / Atom authoring program. If an ATOM file is made available for download from a web page, you need a reading program, a so-called feed reader (aggregator), which then regularly collects the data from the respective page.
Even browsers can be used (sometimes with a plugin) as a feed generator or feed reader.