What is the best way to deal with the main domain (hide or something else), if there are several subdomains and they work successfully?

Question

Search Engine Optimization

Good day everyone. There is a domain and 18 subdomains (by city). What is the best way to deal with the main one, because it is not tied to any city and is essentially not needed, but at the same time you want the best for seo. In this regard, I would like to get answers to several questions:
1. If you hide the main one, will there be a negative impact on the rest?
2. If you make a redirect (main) to some subdomain, and hide it (main) from indexing?
3. And if you still hide, then how best to do it.
4. Your suggestions.

Answer 1

on the main, make a mention of a developed regional network and link to the business as a whole

Answer 2

1. Yes, there will be a negative impact, because although subdomains are perceived by separate entities, they still belong to the main domain.
2. This will also negatively affect all subdomains.
3. The main domain does not need to be hidden. Let me give you an analogy: there is an apartment building. It has 18 apartments, each apartment has its own separate entrance from the street. But the front door is closed. It's just sealed up tight. There is little positive in this.
4. Assign the region to the main domain either "Russia" or "no binding to the region". But specific actions need to be taken based on the niche, site, competitors, etc.