So what do you use turnips for and why does everyone want turnips?
This game is so weird as an outsider. I don’t get it at all.
A character on Sunday sells you a kind of "fruit" called turnips at a fixed price. You then sell these anytime over the next week (before next Sunday, or they'll spoil.) The sell price at Nook's varies twice per day (AM and PM), and also varies between islands. It's possible to make millions of Bells (currency) in profit if your investment is large enough. Most people invest 100-200k in bells every week I think.
So the entire point of the npc selling turnips is so you can buy them and re-sell them at a different time or to people from other islands at a re-sell profit.
And then the people who buy them from your island go and re-sell them to npcs or other players are their re-sell profit?
This game is fucking weird. This is the kind of system I'd expect from a Suda51 game.
Haha.
You basically have it, only thing to note is that you'll usually buy turnips on your own island. There's a slight benefit seeking out the best buy price from your friends, but it's negligible compared to the potential profits of selling.
But yeah, buy low, sell high, it's the stock market just with... turnip stalks.
I suppose in 2020 it does seem like an odd system... The way it works is kind of a by-product of (online) multiplayer being added to the series in Wild World. I suppose in the original GameCube game you could do the two-memory-card thing, but oh man what a pain... I doubt anyone did that for turnips, or if they did, only for 1 or 2 times.

It's honestly not too different a system from how non-native fruit sells for 5x the price. The price just varies day by day instead, and you can only get it from an NPC instead of trees.