Internal links
Internal links create navigation, clarify hierarchy, connect related topics, and help distribute attention and crawl paths across your own site.
Backlinks
External backlinks can send referral traffic, indicate that another site is citing you, and provide signals search engines may use in understanding the web graph.
Why strong sites need both
A page with external links but no internal support can still be hard for users to discover. A well-linked site with no external awareness can still struggle to earn attention in competitive markets.
Fix internal structure first
Before paying for links, make sure important pages are indexable, linked from relevant hubs, included in navigation where appropriate, and supported by useful related content.
Do not manufacture internal links
Internal linking works best when links reflect real topical relationships. Linking every page to every commercial page creates noise rather than clarity.
Important policy context
Google's current spam policies identify buying or selling links for ranking purposes as link spam. Google also says paid advertising or sponsorship links can be legitimate when appropriately qualified with rel="nofollow" or rel="sponsored". Read Google's spam policies.