Because things smell, is everything evaporating?

Not everything is evaporating. You raise the point that we can "smell" certain metals which, given how you smell most things would imply that the metal is evaporating somehow and entering your nose. You'd be right to think this is strange and in contradiction with the idea that metallic bonds tend to be strong and so unlikely to evaporate. This is a good answer explaining why we appear to be able to smell metals.

When a solid or liquid body is immersed in a gas (e.g. ordinary objects sitting in a room full of air) the thermodynamic equilibrium is a dynamic equilibrium in which there is a non-zero vapour pressure of the material of the object in question. That is to say, if you first replaced all the air by clean air, then afterwards the object would start to evaporate or sublimate until a vapour pressure of that material was built up again. This evaporation or sublimation will not go on for ever (unless the air is changed again); it will stop when there is a dynamic equilibrium between material moving between the two phases (solid/gas or liquid/gas).

This allows us to smell most things, as you say. I don't know if there are other issues though. I guess that smell involves some complex chemistry, such that the presence of some molecules is detected indirectly through their effect on others, etc.

Well. "Everything smells" breaks just at consideration of methane. Simple molecule $CH_4$ does not smell. Water $H_2O$ does not smell either. Most gases, in fact, do not smell. "Smelling" tends to be achievable only when reactivity of molecule is high enough. Consider ozone $O_3$, it smells because it is unstable $2O_3 \rightarrow 3O_2$ (the exact reactions are rather complicated but you can refer to corresponding literature about ozone). Chlorine also smells starting from very small concentrations, same with phosphorus.

If we consider organic compounds, mostly we smell what our organs are trained to smell, biologically in the way of evolution. We are good at detecting etheres, but we fail at simple yet multiatomic molecule of naphtalin.