Growing plants is a good example of a middle-voiced phenomenon. It is active, in that there are things that you need to do – water, weed, etc. But on the other hand, it is quite a passive process, waiting for the plants to manufest themselves. This is why Heidegger likes cultivation as a model of a non-technological approach to the world, one that makes clear how active, or activist, or even hyperactive, is any technological approach to food production for instance.
The trick is then to see how a mill and a bridge is like cultivating plants. The analogy suggests that a mill and a bridge that are not technological in their approach to a river are ones that work with the river in its way of presencing. The mill makes use of and even draws attention to the flow of the river, not perverting that flow into something completely alien to the flow, like electricity, but just amplifying the flow into a turning, the kind of grinding that a river already does to the pebbles that make up the river bed. A bridge is perhaps less straightforward, in that a bridge seems to do the opposite of a river, staying still, spanning the air, etc. However, Heidegger talks about the way some designs of bridges draw attention to the solidity of the banks of the river, a solidity that allows the river to flow within those banks; similarly the spanning can draw attention to what flows under the span. Or at least, you can probably imagine a design of a bridge that is really inappropriate to nature of the river, its banks and flow; such a bridge probably looks quite engineering-y, a kind a technological plonk onto a landscape.
Importantly, these examples are not really issues of control. Heidegger is not like other philosophers of technology who worry that technology is out of (our) control. Heidegger is more interested in explaining how technology is a form of presencing, a bringing forth. It is one that is very limited in scope, limiting things to being present merely as resources, as things that have no inherent nature, but are just means for other things. But it is nevertheless one amongst other ways in which things manifest. And as such it is never entirely controlled, by humans or nature. The issue is not who or what is controlling, but what things get to manifest as.