In the John Mueller 09/17/2021 Q&A, one webmaster was concerned about subdomains vs. subfolders.
John thought someone was trying to troll them again with this question.
John explained that subdomains and subdirectories are equivalent. It doesn’t matter which structure you use to create your website.
It’s possible to put your content together however you want.
In Google’s mind, it doesn’t matter either way.
John does, however, mention that there are several aspects that play a role that have less to do with SEO and are more about how you set up your site. Is it easy to set it up? Is it possible to track the performance of the structure you use on separate hostnames? Together on one hostname?
For some sites, it’s possible that Google might treat something on a subdomain differently. They may think it’s a separate website vs. all of the same website.
All of these aspects could play a role here.
This discussion occurs at approximately 45:26 in the video.