Hello, i read online that google treats a subdomain as a separate website, so i want to know, if i have a website at example.com, and i got hit on google, like my ranking dropped, would it also affect new subdomain created after the hit? like finance.example.com.
According to the cases I've found over the past years, subdomains are not treated as a separate website in all cases. There are two types of subdomains: the massive ones, also called web 2.0 (like wordpress.com, blogspot.com) and the regular ones, that are the ones that we generally create.
For example, I have many sites with subdomains per language like fr.example.com de.example.com ... and they are all treated as one domain.
It's true that the reputation is not 100% shared between the domain and the subdomains and I have not found an stable relationship. But there are huge relations, and given that the footprint is 100%, I would never build over a hit domain through a subdomain. It is really risky in all senses.
As a rule of thumb, try always to simplify things. Don't overcomplicate and go through the most straight path possible. Maybe there is a use case where it would be interesting to do a subdomain, but if you are doing this to save the cost of a new domain, don't.
Anyway, be aware, that you can probably continue in the same domain hit by the ranks, and recover. Unless your site was dirty spam (like a download site filled with ads)