The structure in a spreadsheet is “all the stuff is in cells” and in a raw binary file it’s “ all the stuff is in a binary blob” in a txt file it’s “all the stuff is in lines in a file”.

As I wrote initially, if you losen the definition that much, then a txt and literally everything is a database.

There are many different definitions but my personal completely subjective perspective is that the bare minimum is that whatever structure you claim the data has must be enforced and be translatable to a semantic connection. For most sql databases you have a schema enforcing the structure and at a minimum there is a connection between different pieces of data in the same row.

In a spreadsheet having different sets of numbers/text/code on the same row or column might mean something but doesn’t have to. So whatever “structure” there is, is not enforced.

This is in the same sense that a txt document that contains 4 book chapters have in them 3 email addresses is a “structured list of emails” sure you could find them and there is a structure to it, but calling it a database is a stretch in my book.