One thing that seems popular right now is group giveaways, where the goal isn't to get readers/sales directly through the giveaway - the goal is to get many people to click on the authors' instagram/amazon/twitter/twitch/tiktok/etc. pages. Enough views trick the algorithm into thinking the page is popular, and then it shows the page to people who might actually be into it. Or at least that's how it was explained to me.
The way the giveaway works is that you get sweepstakes tickets for each action - for clicking on each author's insta page, on each author's amazon page, for liking an insta post, for following on twitter etc. It doesn't matter if people are not actually interested or unfollow later - the algorithm boost is the goal.
These giveaways are advertised in various places, often to people who don't even read that genre. Only the prize of the giveaway needs to be something the targeted group is interested in - like a Switch or Playstation if you target gamers, or a voucher for some online clothes shop if you target that demographic. Or cash, or paypal/amazon gift cards.
My sister won a $500 gift card last month and she doesn't even remember whose sweepstakes/giveaway it was. "Some twitch streamer or something" apparently.