TXT are so-called plain text files, so simple text. Normally these are sequential strings of representable characters. Control characters are used for structuring. Mostly an ASCII or Unicode encoding is used. Such documents can be opened and viewed by any word processing program or text editor. Even a browser is able to display such files.
However, it should be noted that very simple as well as slightly older text editors may not be able to handle all encodings and thus, for example, German umlauts encoded in UTF-8 can not display properly.
It is also important, for example, that texts are generated for web pages in the coding, which should later also use the website.