Azek trim on bay window box

I have a bay window with a (I think) classic 3-sided trim bottom. The trim is rotting and I plan on replacing it down to the studs and back. So studs, exterior ply, tyvex and flashing, then trim – either poplar or azek pvc.

Here is the first ask – could you take a look at this video?

https://solutions.dunnlumber.com/projects/using-and-installing-azek-trim

Note how he creates the box structure – he has an outer rectangle of trim over another panel then insets trim to create a nice 3d effect.

My question, and I looked throughout the article, is what is the first layer? From the video I can see he put down his ply and tyvex but for the first panel, is that azek so he is nailing azek to azek? I wasn’t sure you could do that – I thought it was azek to substrate (ply).

I’ll try to ascii draw it.

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|                                  |
|       -------------------        |
|       |xxxxxxxxxxxxxxxxx |       |
|       |x               x |       |
|       |x               x |       |
|       |xxxxxxxxxxxxxxxxx |       |
|       -------------------        |
|                                  |
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In this “drawing” the x is the decorative trim, the box it is in is the inset, and the outer. So the stuff x is attached to, probably azek?

Trying to figure this out so that I can make sure I have enough room as my drip edge fro mthe windows above is only so big.

Thanks.

Meta: Also, if someone with 300+ rep could add the tag “azek” I think that would be useful for others.