qgis - How to add my own symbols to the single-marker or svg-marker selection list?

I was creating an arrow for a line layer. So I was adding the triangle as a single marker to the line. So far so good. But when I would like to create my own single-marker symbol or svg marker how can I add these own symbols to QGIS? Where have these own symbols to be stored? So that they are added to the single marker or svg marker selection list?

I could not found the folder by searching for .svg files? ;( What's the approach to put my own single marker or svg marker to the selection list?