How does this situation affect SEO?

Question

Search Engine Optimization

We have registered one domain that hosts the main site.
At the same time, several more sites were created, but we did not register a separate domain for them, but we place them on a third-level domain from the main site, that is, newsite .mainsite.ru.
At the same time, the sites themselves do not intersect with each other in any way, they have different content, and in fact, everything that unites them is a domain.
Tell me, is this very bad for SEO (in relation to the main site), or is it okay and you can ignore it at all?

Answer 1

This is bad only from the side that if you transfer sites from subdomains, then they will be counted by search engines as duplicates. And they won't get all the informational weight they deserve. In this case, use redirects.