If you see little DNA icons on people's trees, it doesn't mean that they have attached a DNA match to their tree - that is not made visible to other people. It will just mean that they attached an image for whatever reason.
I can't find info on Ancestry about how to connect a DNA match to your tree but here is a blog post about it (you need to scroll down a bit because it does FamilyTreeDNA first):
https://www.yourdnaguide.com/ydgblog...o-family-trees