Influencer marketing = reach + content. You pay an influencer to create content and share it with their own audience. You are buying their reach and influence to reach their followers directly.
UGC (user-generated content) = purely content. You have content made by consumers or dedicated UGC creators. They do not post it on their own channels. You buy the content (image rights) to post it organically yourself or to use it as a credible advert (social ads) on your own brand's channels.