Platform.sh templates helps to create D8 multisite projects
https://platform.sh/product/demos/drupal-multisite/
https://platform.sh/blog/2020/multisite-on-platform.sh-the-how-and-why/
The example shows how to setup the multisite using subdomain
How can we do this using one domain & with folders (subdirectories)
www.example.com
www.example.com/site2
www.example.com/site3
in D7 example would be this
https://www.drupal.org/docs/7/multisite-drupal/multi-site-in-subdirectories
Have created a query in the github project as well
https://github.com/platformsh-templates/drupal8-multisite/issues/24