system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Who/how do horoscopes get put together each day?

I have a horoscope app, everyday they come out with new and different horoscopes.  Most of the time they are very specific.  How do they come up with ths stuff?
gpt
This is known as the Forer Effect.  Other names are Barnum Statements and Cold Readings.

So what are they?  Basically, statements which are worded in a way that make you believe that they apply to you.  If you were to be given horoscope paragraphs without context (zodiac sign, month) then you would assume that the statements are just as accurate.  

The story you will find on Wikipedia is that a psychologist named Forer once gave a personality test to his students and asked them to rate how well they thought it applied to themselves.  He gave them all the exact same paragraph and they rated it highly.

The eventual conclusion is that you lend credibility to the horoscope by assuming that the horoscope provider is of a good authority, and it is bolstered by you wanting to believe in the horoscopes.  

To answer your question directly - now that you know how these horoscopes work, you can see that it is easy enough to construct enough horoscopes and randomize them over a period of time and assign them to various signs.