Online slots evolved from simple three-reel games to a broad range of styles...

https://thefiguremagazine.com/classic-reels-to-megaways-how-online-slots-became-more-varied/

Online slots evolved from simple three-reel games to a broad range of styles that include themed video slots, Megaways with dynamic paylines, bonus rounds, jackpots, and Slingo hybrids. These options add variety and different pacing to gameplay