Sorry, but this isn't a question we can answer - and there are several reasons.
Firstly .TXT files don't have a structure: they are just text and you can store any form of text in them, but the text they contain may have structure if the application (or user) who created them applied some to the actual data. For example, you could store a whole book in a TXT file, and orginse it into chapters, pages and paragraphs. Or you could store data separated by commas and using lines to delimit rows.
Even then, the .TXT extension doesn't mean that you can read it - extensions aren't "fixed" and any application can create a file with any extension regardless of the content.
And worse, .DOC is a common extension that could refer to any of hundreds of applicatiosn data, but even produced by Microsoft Word, there have been a number of different (and incompatible) versions of the actual content - so there is no ".DOC" file format that is guaranteed to work with all files.
If you need to know about a specific file format, then start here:
List of file formats - Wikipedia, the free encyclopedia[
^] - but don't expect it to be a trivial matter to process them yourself!
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