I like the look of horizontal boards to enclose the deck. You could use wood and have an opaque stain color matched to the Trex color.

Following

Same way as the sample that houssaon provided. You don't want to mix wood with composite deck structure it will look out of place.

You can also use lattice with 1.5" reveal Example Here, you can use Trex Decking material Example Here.

You will also have to recut new stair stringers, and most likely add additional 2 stringers (depending on the width of the stair) to break up the span, composite should have stringers spaced 12" on center.Example Here

Thanks for the comments so far!

can I use trex fascia boards as skirting? for a look like this? Will this work on my deck? (pics of my current deck in my original post)

You can just keep in mind that the facia board is not cheap and it only comes in 12' lengths so you could end up with a lot of waste if you don't plan accordingly.

ok! So what’s the best trex product to use for thre vertical skirt? Just the normal deck boards

If you want to close up and you want to use decking, buy cheaper Trex boards with scalloped backing instead and you can get them in a different color if you like to add some detail, installed vertically they will not get much exposure and retain its color.

Thanks - I also came upon another potential solution called deckall mesh skirting - ever heard of this or used this?

Another idea would be to incorporate landscaping. We installed trellises and planted pink jasmine to grow a “wall” from ground to deck. It creates a lovely backdrop for other plantings, and the fragrance is a sweet addition to summer evenings on the deck.