> how "renewable" are the materials used to produce these

Very renewable. Solar panels are mostly glass, silicon and a little bit of metal. And they last ~30 years. Wind turbine blades are made out of fiberglass or similar materials. They may need replacing every ~30 years as well.

Other infrastructure would not need any significant maintenance for even longer.

These kind of power plants, apart from being renewable, have very low running costs. And that is the point.

Of course their production is very variable and therefore they cannot be used as the only power source. So e.g. nuclear power plants are still needed to back them up.

I think it is very rational to build as much power plants that are cheap to run. And back it up with nuclear or other power plants that are expensive to run but which can cover for time when the production of renewables is low.

Mono-crystalline silicon - which is now the dominant technology - is a pretty clean, but thin film PV - which is on the wane - had high heavy metal content. Good news.