SEO is split into two parts on-site and off-site in this on-site SEO is the most important one among them.
This on-site SEO can be separated into two based on their character that is content-related and code-related.
Writing semantic codes, building user-friendly website structures are all code-related.
Content-related is based on contents like description meta tags and so on.
The on-site and on-page SEO are the same except for one difference on-site SEO inclines to build whole site structure, especially internal links between different pages on the site.