Why internal links matter
Links between your own pages help search engines discover pages and understand which ones matter most. Pages with no internal links pointing to them (orphan pages) are often crawled less and rank worse.
Link to related pages with descriptive anchor text: "STL to STEP converter" tells Google more than "click here".
External links
Linking out to helpful sources is normal and good for visitors. Mark paid links as rel="sponsored" and user-submitted links as rel="ugc".