Redirecting your .html site to WordPress is just like redirecting any other web page or blog post. In order to avoid losing your Google rankings make sure that you apply 301 Redirect (permanent redirect rule).
For example if your html page is example.com/post.html then it should be redirected to WordPress post that looks like this: example.com/post
If you are able to make sure that all your pages have a 301 redirect then Google will detect the changes and pass the rankings to the new site
Keep the 301 redirect for at least one year
Google has provided more guidelines on how to change site URL in this page: https://developers.google.com/search/docs/advanced/crawling/site-move-with-url-changes