Improvements on a single page of a website do not result in an overall higher ranking of the website by Google. However, they can result in better rankings for the page in question.
Google’s assessment of the quality of a website influences the overall rankings of the website. This becomes particularly clear with Google Core Updates: This is about the quality and relevance of websites as a whole. If there are qualitative problems, there may be a risk of ranking losses. If, on the other hand, work has been done to improve the quality of a website, a core update can also bring about significant improvements in the rankings.
However, improvements to one or individual pages are not enough to improve the overall quality rating of a website by Google. As John Mueller explained in the June 11 Google Search Central SEO Office Hours, content is indexed and ranked page by page. On the other hand, there are also signals that cannot be evaluated at the level of individual pages. This is where Google needs a better understanding of the entire site. This also includes quality. Significantly improving the quality of a single page will help that one page and its rankings. But it takes longer for this to affect the quality rating for the entire website.
An example is websites that previously had a lot of adult content, for example in a forum. If you gradually remove this content, Google will recognize this for individual pages over time, but not for the entire website. Eventually, Google would then realize that the improvements would apply to most of the site and stop Safe Search filtering the entire site.
So, improvements to a single page don’t add that much to the overall score of the site. However, incremental improvements can lead to a better rating of the website by Google after a certain period of time, which can then be expressed in more noticeable improvements in the rankings.