Ask HN: Links changed, SEO help

I have a small site where I teach people to code by building projects (https://enlight.ml)

Recently, the links for each page changed as I redesigned the site form the ground up. However, Google still isn't re-indexing the pages with the new links. In fact, google indexed the pages with new content - but ignores the new link for the old content. It still has the old links appearing in search results.

How can this be fixed? I tried submitting a sitemap and submitting the new links several times in the Search Console.

Is there a way to do redirects on sites hosted GitHub Pages?

Lastly, I need some SEO help. Aside from providing good content, how do page rankings work? My goal is for anyone to be able to find a project that they would be interested in easily if I was on the frontpage of that search.

Appreciate all your help :)