Domain Authority

Quality "non-spam" Do-follow backlinks, more posts with higher search ranking, good traffic .. This should help

The more months and years goes with some content on the blog (blog age, popularity and content frequency) then DA will increase automatically regardless of the custom domain or blogspot/wordpress etc.

For example, if you have a new custom domain of a year then it will be around 10 to 15 DA and if you have four or five years blogspot or wordpress blog then the DA will be around 40-50 or even more. They changes the DA algo after five years and on. It may be funny too.

This algorithm was once matching with mega search engine like Google but Google gone far ahead on these algo.

I slightly differ here to HR 's point.  Free blogs with .wordpress and .blogspot , the DA is not applicable.

Because even for a relatively new blog the DA might be around 65 which is actually that of blogspot.  So to simply put, keep posting good content, share it, write guest posts , get do-follow links and it should automatically improve yours

Slightly or wholly you can differ .

DA is generally applicable for all blogs. You mean the second onward blogs from the very same account and its rank. Yes, it means there is flaw in DA ranking for those WP/BS, so, they have to fix it. In short, DA generally works based on the 'Age' factor first and the other factors later - it is not as we see many articles around on this topic. DA is just a part of SEO.

Keep posting content frequently and build backlinks with mix of nofollow & dofollow (60:40 Ratio). Also, don't ignore social platforms as it'll be one of the stong source of traffic and helps to increase DA of your website in majority.

Simple and yet difficult.

Focus on content and marketing, be regular at both, analyze and course correct and keep doing it for years, everything else will follow.

Building quality backlinks is the best bet.

I've managed to get myself a sizeable amount of quality backlinks from my employer (an online magazine) where every  byline of mine gives me a link back to the blog.

Guest-posting with a backlink usually works.