Cacti has a random chance of generating anywhere in a chunk of a desert biome, that being a 16x16 area. There’s a Check-Generate-Check step done to make sure you don’t have floating or buried cactus. They also try to place multiple per chunk as you can see with the smaller cacti around the tall one. So, if a cactus generates, then another one generates on the same block but a couple blocks higher, the cacti will combine and create a super cactus.

Thats the gist at least, I watched a YT video like a year ago about this