I'm not really satisfied with my earlier analogy with farming and mining, mostly because as I delved into them the distinction I was trying to make didn't seem as clear as I thought it did.
Perhaps I was trying too hard to force it to fit.
Stepping away from that for a second, I think what I want to envision is more like this:
Picture one of those lazy rivers, where the water loops around and people float along in inner tubes.
Now, imagine a much bigger and more intricate one, where water may flow into another channel where it turns a water wheel before returning to the main loop.
Now, imagine that this isn't something that was built with water proof walls. Some of the water sometimes seeps into the ground, and flows out.
Also, there are springs in various places where water comes out and adds to the volume of water.
To bring this back to my little economic image - some things are springs, and add volume to the water flowing through the system. Some things are water features that redirect the existing flow. And sometimes, some of the water seeps out into the ground and lessens the volume of water.
It is not quite a closed loop, but we want the flow to circulate around continuously. Most of the volume of water loops around. For the economy to grow, though, we want more springs adding to the volume of water. And we want to reduce the places where water seeps out of the system.
For my farming and mining analogy, when I started it I was basically imagining the typical 'someone gets isekai'd to a wilderness area trope and thinking of how they survive if they can't immediately find civilization.
You basically have to secure water, shelter, and food right away. Generally that means trying to create tools out of anything in your vicinity so that you can hunt and gather food. Farming generally comes next, if you can. Mining is... useful and important for making life easier, but not something you really focus on before food and shelter. At least, not unless you're taking advantage of resources that are easily visible.
It's true that in a modern economy there's startup costs to farming - the cost of the land especially, but also seed, water, and farming equipment.
So in thinking about it more, what I want to distinguish between the circulation of the existing volume of 'water', things that add more 'water' to the flow (like a spring), and things that reduce the volume of 'water' (like dehydration or ground seepage) - where water can sometimes mean the flow of currency, but it's not strictly limited to money because this would also work in a barter based economy. So maybe the flow of economic value? I think I'll leave that to the economists.
I also want to point out that some things occur 'downstream', or later on in the economic flow. Mining is more downstream than farming, since it relies on having a labor pool that doesn't have to spend all it's time farming. Plus having the resources to find veins, dig into the ground for them, etc.
Whether it's a 'spring' or just a water feature that moves around the existing flow depends. I don't know if there's a way to measure the net volume, to see what goes in and what comes out or something...
Again, you'd probably want to talk to a real economist as I'm sure there's somebody out there who's already thought of this.
I suspect the initial raw goods of food production act as springs, overall. I am less certain about all the 'downstream' products that depend on earlier flows.
