I know that, with custom scripting enabled, you can save a SharePoint Online site as a template and then download the
.wsp solution to your computer. I was wondering if this was possible with a built-in template, such as
Actually, I don't know exactly what you need to do to can help you further!
But if you are talking about accessing the default template physical path
C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\TEMPLATE\SiteTemplates\sts that contains the site template files, so it's not possible to do that in SPO.
However, if you are thinking to create your own template that is identical to the built-in template like STS#0 and you would like to generate a WSP solution for that template,
So you can simply do that by creating a new site from this build-in template without doing any kind of customization, then save this site as a template!
In this case, you have already a
wsp solution for your own site template that is similar to the built-in template.
Check also, Templates in SharePoint Online
I have tested with
STS#0 template and it is supported to be saved as a template and download to local drives. (I cannot test with
WIKI#0 since this template is not available in my tenant.)
If you cannot find the Save site as template option with custom script enabled in this site, you can use the following URL to redirect to the page: