These two activities are generally misunderstood or used alternatively.
On-Page SEO includes all the activities that are related to content. Such as page or site content, title tags & meta tag optimization, Heading tag optimization, internal links, image optimization, etc.
While on the other hand, Technical SEO activities include improving site speed, creating and submitting a sitemap, optimizing your website for structured data, etc.
On-page SEO activities help the search engines understand your content better and rank it accordingly.
And technical SEO activities impact your website indexing and crawling by the search engine.