A sill is the exterior bottom of a window frame so It comes with the window. The interior bottom molding is added after the drywall is installed and called a stool and the molding below it is called an apron. Just to make it difficult (I guess) some people insist on calling the interior molding a sill. I suspect the interior trim (and perhaps the jamb and head trim too) is a hybrid material like wood fiber and PVC, etc. When painted it looks like wood. You need to ask the builder what it is but don't be surprised if he tells you the brand name instead of what its made of.

Some are cellular PVC. Some are wood fiber and resins. Some could be super kiln dried lumber. The only way to know the specific product is to ask the one purchasing it.

Could be an MDF product as well.

There are a lot of good composite materials they could use, and most will hold up much better than wood. However, it would be good to know exactly what they are using.

For example, I assume James Hardie products (cement board) would fit into that category, and we also used a compressed coal ash product made by Boral for all our window trim.

Bruce

I can't imagine why a fake grain embossed cement or fly-ash composite would be appropriate for interior moldings. Are they even made in shapes?

A painted window stool needs to withstand sunlight and water. I would not specify anything less dense than poplar. Its not the place to be cheap.

My stools are wood - every house that I can think of. And then painted (or stained).

My sills are usually composite or vinyl or wood.

But, I think we can all suspect the OP is talking stool and not sill. This is a language switch that I do all the time unless talking to a builder/contractor. Common English as spoken in my area of the US has the stool being called a sill.

Composites are like sausage. Depends on the source.