Does SEO benefit from adding subdomains to a website?

Does SEO benefit from adding subdomains to a website?

If I tell my experience, then adding all the domains in the website does not benefit or harm seo.

Because when you create a website from a subdomain, it is a completely new website in the eyes of Google and you have to seo that website again from beginning to end.

But if you create a new website from all directories, then there is no loss of your seo in it, meaning you do not need to do seo of that website properly.

But if the experts agree, then of course you can create a new website from all domains or from all the directories, you should seo your new website from the beginning.

Because this also improves the seo of your main blog.