Andersen's recommended stool detail is different for the 200 Tilt Wash double-hung, the 200 Narrowline double-hung and the 200 Casement/Awning windows. They are all shown on their website. If you tell me what kind of window I can post the right one but the basic answer is that the stool would be applied over any extension piece and the apron would be below that.

I have mostly 200 tilt wash, one casement and one awning. Do these go over the extension jamb or do the take the place of the lower extension jamb? It doesn't look like there is enough room between the bottom of the sash and the top of the extension jamb for anything to go on top. Maybe 1/2 inch.

The stool trim goes on top of the extension piece and it should overlap edge of the sash creating a better weather seal in the traditional manner.

Be very careful how you install these windows because (unlike all the other Andersen double-hungs) there is no slope or drip at the exterior sill and water ha a tendency to run over and under the nosing and get behind the siding. Sealant at this joint is only temporary. It is often necessary to add flashing that starts under the sill nailing fin and terminates between a course of the siding. I've seen it done retroactively to hundreds of these windows.

Another option is to add a PVC sill nosing from ATW as linked below. This would give the 200 Tilt-Wash a similar drip nosing to the other Andersen double-hung windows (400 series & 200 Narrowline) and allow the sill to extend at each side under the jamb trim in the traditional manner.

For a PDF of installation details go to the Andersen website, select the 400 Tilt Wash, then select "for Professionals" and then select "architectural details and product applications".

Here is a link that might be useful: Sill nosing trim