This is a commonly repeated factoid, but it's not really true. Or at least, it's very misleading. Let me explain.
Rural populations tend to be older. That means more spent on Social Security, more spent on Medicare, and so on. This is by far the largest reason why people say that cities "subsidize" rural areas. But if those old people moved to cities, they would still be receiving Social Security and Medicaid. So it would be more accurate to say that young people subsidize old people.
Farm subsidies exist and probably should be reduced (or eliminated) but in the grand scheme of things, we're not talking about much money. It was about 22 billion in 2019, according to NPR [https://www.npr.org/sections/thesalt/2019/12/31/790261705/fa...]. That's comparable to some of the increases in yearly budget that are being talked about for Amtrak, a program that mainly benefits cities.
So don't move to a rural area believing that the federal government will shower you with cash. In general, it won't, or at least not more than it does for city dwellers.
On the other hand, being old or poor may cause the Feds to shower you with cash, but not more than you would have gotten in a city (and many things will be more expensive in a rural area than a city).