Internal linking is the most underrated thing you control
Links from other sites require persuasion. Rankings require Google's agreement. Internal links require nothing but a decision, which makes them the highest-leverage work available on almost every site.
Links tell Google what a page is about
Anchor text on internal links is one of the clearest signals available about a page's subject, and unlike external anchor text you control it completely. A page linked from thirty other pages with vague anchor text is being described vaguely.
Use descriptive anchors. Not 'click here', not the bare URL, and not the same exact phrase every time.
Depth is a ranking problem
A page five clicks from the homepage is being told it is unimportant. On large sites this happens silently as content accumulates and old pages sink beneath pagination.
Run a crawl, sort by click depth, and look at what is buried. Commercially important pages sitting at depth four or five are the easiest wins on most sites.
Link to the pages just outside the top ten
Pages at positions eleven to twenty are already close. They usually need a small authority push rather than new content, and internal links are the cheapest way to provide it.
Find relevant existing pages with authority, add contextual links with sensible anchors, and check again in a month. This is unglamorous and it works more often than not.