Also consider that DA is for the domain, so if you register a subdomain in a domain with a given DA, you preserve the DA (but doesn't mean that your subdomain could be useful at all, it's just how the metric works).
But apart from links as you have read, there is no way to rise the DA. Links don't have to be created by you. There are thousands of high DA shitty, pages that are constantly linking other pages for the sake of doing such (statistic pages, negative SEO pages, weird image indexation pages, etc..) But it's all about links ultimately.