How to combine subdomains with the main domain for search engines?

Question

Domain name market

There is a site in which all sections are placed on subdomains. I.e .:
site.com - main page with links to sections
forum.site.com - forum
chat.site.com - chat
photo.site.com - photos
, etc.
The problem is that the same Google considers them to be different sites and thinks that there is no content on the main site. It is necessary that he understand that it is all a single whole. Is there any way to explain this to him?

Answer 1

Why should he understand that this is a single whole then? When it is at least not the same.

Answer 2

A 3rd level domain is just like any other domain. Domains are not combined with each other, except by the principle of mirrors, i.e. absolutely identical in content. So it was a few years ago when I clarified this issue. Maybe things have changed now, but I doubt it. If wrong, correct...

Answer 3

Recently, at the Yandex webmaster conference, they said that this is possible through mirrors. Moreover, it is desirable to put a webmaster and track whether the search engine understands that the sites are mirrors or not. And this is a long process - 2-3 months depending on the size of the site.
events.yandex.ru/events/yamasterskaya/

Answer 4

I received a letter about a change by Google just in this direction - the rejection of subdomains, look