How do some sub domians have the same authority with main site?

I was wondering how can some sub domains have the same domain authority with the main site? for example aws.amazon.com has the same domain authority with amazon.com but a blog made by wordpress.com like nima854007256.wordpress.com which is a sub domain of wordpress.com doesn't have the same domain authority with wordpress.com. and how to make a subdomain that has the same domain authority with main site?