The difference is not follower count, it is who posts and who owns the content. An influencer posts to their own audience, so you pay for their reach and trust. A UGC creator makes content the brand posts and runs in its own ads, so you can have zero followers and still get hired. UGC pays per video and is steadier, influencing pays more once your audience is big. Plenty of people do both. Start with UGC if you have no following yet.
Usage, not size
UGC content runs on the brand channels. Influencer content runs on yours.
Followers are optional for UGC
Brands hire UGC creators for the content, not the audience. Zero followers is fine.
Different pay models
UGC pays per video and is steady. Influencing scales with your reach over time.
You can do both
Many creators run UGC for cash flow and build a following for bigger deals.