@hannahshrontz  That's a really good question!  I also did not realize the Sunday Riley products contained silicones.

I did some quick research, as I'm interested in this information also, and I ran across a site called "beautifulwithbrains."  The author has an article called, "Do Silicones Prevent Other Ingredients from Sinking into the Skin?"

In this article, the author, Gio, quotes Paula from Paula's choice as stating the silicones work like a net or a tea bag; they are porous and allow other substances to seep through to the skin while helping the skin to retain moisture.

There is a lot of good information in that article- Sephora doesn't allow links or else I would post it here, but it's definitely worth reading as it will answer your questions.