subdomains dont gather any age, ideally you want a sub folder... subdomains are independant from the site in a way, but then there are factors which can make a sub domain stronger, like if its hosted in your country etc.
You can get sub domain back link but if you have chance to get main domain back link so this back link will work better than sub domain
It really doesn’t matter whether the backlinks come from a domain or subdomain because what really matters is the quality of the page the link is on. If the link is on a page which is poorly ranked or irrelevant then that link will not hold any significance whether it is on domain or sub domain. But when it comes to rankings, Google will prefer link from a domain as compared to a sub domain. Quality of page decides the strength of backlinks, not domains.
the answer is yes. As long as the page with quality content, it doesnt matter it is subdomain or main domain
Quality content is important, domain and subdomain is not important. if the page having good quality content, then definitely that page is valuable.