They are document icons showing that they have sub-documents. That’s all.
You are sering them.
If you look in the Windows file browser, you’ll see these disclosure triangles when you hover over the side-bar where folders can be expanded by clicking on them.
Hover your mouse over “This PC”, or the “Documents” folder and you’ll see them if they’re not visible all the time for your PC’s settings.
To make them more visible, you’ll have to research how to adjust Windows’ dark mode colors so that the disclosure triangles are more visible. In my cursory research, I couldn’t find a basic Windows 10 setting that let me do that.