At the moment, none of the pages link to any other pages and none link to them. If you make the calculation once for each page, you’ll find that each of them ends up with a PageRank of 0.15. No matter how many iterations you run, each page’s PageRank remains at 0.15. The total PageRank in the site = 0.45, whereas it could be 3. The site is seriously wasting most of its potential PageRank.
This is, in fact, the most common question that people ask a webmaster. I have put together a comprehensive article which explains how does a page ranking algorithm works in Google. You can read the article here. This article helps a new user as well as experienced user to pump up the page ranking via amplifying page ranks. Google page rank understanding? SEO 2019 | ShutterholicTV SEO Guide
A variety of methods can increase the prominence of a webpage within the search results. Cross linking between pages of the same website to provide more links to important pages may improve its visibility. Writing content that includes frequently searched keyword phrase, so as to be relevant to a wide variety of search queries will tend to increase traffic. Updating content so as to keep search engines crawling back frequently can give additional weight to a site. Adding relevant keywords to a web page's meta data, including the title tag and meta description, will tend to improve the relevancy of a site's search listings, thus increasing traffic. URL normalization of web pages accessible via multiple urls, using the canonical link element or via 301 redirects can help make sure links to different versions of the url all count towards the page's link popularity score.