Yes, Sam, the initial page, if one existed, would be at /$serverroot/www/index.*
but since there are no files named index.* there, the server's "request processor" makes a page based on the requested url. 'mydomain.tld:8080/' would be mapped to the default subsite index page files at:
/$serverroot/packages/acs-subsite/www/index.*
which are a file pair, index.adp and index.tcl
The details of the request processor behavior can be read at:
https://openacs.org/doc/openacs-5-1/rp-design.html
https://openacs.org/doc/openacs-5-1/request-processor.html
As an example, one can make a custom first page, based on the template acs-subsite index pages by copying the index.tcl and index.adp page to /$serverroot/www/. and modifying them there.
The templating system has useful features to help build dynamic pages:
https://openacs.org/doc/acs-templating/
Hope this helps,
Torben
