1- First for the "on page" SEO?
There are many factors within the site which helps you to improve the appearance of your site or improving google "Ranking" of your site whether the factors related to the content of your website or your for example the title of the site, the speed of the site, the content of the site, the size of images and videos on the site etc ..
you can do this by Google Chrome open your website and in google chrome and audit your website
it will fix your website for any problems
It will show you how to fix any problems within your content and therfore you will solve many problems in your website .
2) Second :the Off-page "off page" SEO?