Template Studio - Is there a tutorial available for creation of .xaml files for symbol design?

I am trying to add new symbols and revise old ones. I saw in https://communities.bentley.com/products/geotechnical1/f/openground-forum/215971/importing-new-point-symbols that Bentley will create symbols for you upon request.

I want to be able to make minor/precise revisions to the symbols myself. Is there a tutorial for this, or is there a way to download the .xaml files being used for the default symbol types?

Thanks,