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.
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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.