They pay small amount to influencers for promoting their accounts. And the owners of private accounts , presumably, use SMM panels to boost the specific video. In such way they have a lesser chance of getting banned/shadowbanned since the main source of followers come from other accounts promoting them on their videos not from their own videos. If you ask me this is kinda smart - create organic buzz around yourself and while initially post content that might seem private or creepy, you can use it afterwards for CPA or promoting offers. A big deal here is that they have verified accounts, therefore creating as much credibility as possible with a private account. However this method requires some initial investment - verified accounts usually go for around $1-2k and paying to influencers and SMM panels might be extra.