Is it good or do they just have a massive network and data advantage. If tik tok left and everyone switched over to Instagram reels or YouTube shorts and they had the same amount of data tik tok has I think the experience would converge to whatever was on tik tok in a month or so.

There's no secret sauce to tik tok, they're throwing massive amounts of data at a recommendation AI and telling it to optimize for watch time, any sufficiently scaled company can do that nowadays. It's more a matter of getting and maintaining an audience to create that data and content creators, both of which due to the network effect, and without federation, are drawn to the biggest service, not necessarily to the best.