It depends on the brand’s budget, but my company uses influencers in 2 ways - 1) our marketing team send out free product or gift cards in the hope that the influencer/celebrity wears/mentions said product or 2) if there’s a new campaign for example, we hire influencers to promote the product with specific deliverables (i.e., 2 IG posts and 1 reel) and they are expected to follow a brief. If the budget is big for a new launch, we may hire a few bigger influencers and then send out product to smaller ones, or it could just be one big campaign with a celeb and then paying influencers to promote the product. And then influencers are entitled to the affliate income.
I’ve heard from colleagues that H&M has a huge marketing and influencer budget, which is why it seems like they work with everyone, plus there’s a lot of affiliate money. And the ROI for influencers themselves must be huge because rarely do you see an influencer do one campaign with H&M and then nothing.