Its probably some combination of paid "shout outs" from popular accounts to the new account they are attempting to build and reposted or shared content where the popular account showcases a piece of content from the new account.

There are also likely to be large, influential, account networks that are real and when used to boost a new account, very effective (I know numerous people on instagram with multiple million+ follower accounts).

All of this is similar to content marketing and link building strategies that have been effective for a decade plus.