Well, the difference between ND and NT is, on paper, quite clear.
"Neurodivergent" is an umbrella term, and typically covers autism, ADHD, dyslexia, dyspraxia, dyscalculia, dysgraphia, sensory processing disorder, and Tourette's among other rarer neurodevelopmental disorders. Some also include mental illnesses under that umbrella, particularly lifelong conditions such as bipolar disorder, schizophrenia, OCD, and eating disorders; the conditions that seem to straddle the gap between "neurodevlopmental disorder" and "mental illness."
In practice, the difference isn't always so clear. Of course, many people are undiagnosed. Even more are neurodivergent but don't really think about it, especially those who have received good support for one of the more "situational" conditions such as dyslexia.* I also think a lot of people also see content online which is intended by neurodivergent people for neurodivergent people, and assume that because they relate to the topic of a few videos that therefore "everyone is neurodivergent these days."
When you see a video you relate to, it's important to remember that one of the common diagnostic criteria for conditions like autism and ADHD is that symptoms must be lifelong, and must significantly impair your quality of life. Losing your keys every now and then is one thing, but losing your keys several times a day to the extent that you're in tears over it? That's the problem. My autism and ADHD symptoms are not dependent on stress or fatigue, and they're not just something I have on an "off day." I regularly forget important appointments, or I'm too anxious about talking to people to leave the house, or I have to set alarms to remind me to eat, to take my medication, or to communicate with my family. Many of the videos people seem to be talking about are like those I see from neurodivergent creators, in which they are laughing about the funnier aspects or commiserating about the little inconveniences amongst an intended audience of likewise neurodivergent watchers. They're not showing their worst moments, in the same way I never tell my friends about my meltdowns; it feels shameful, and ugly, and private.
I'm having a difficult day getting my brain to work, so I don't think I've managed to express as well as I'd like. At least I hope someone gets something out of it.
*I know that dyslexia can actually have a wide variety of symptoms besides the well-known issues with reading and writing, such as executive dysfunction, working memory, and spacial awareness, and that for many people it is not just a situation difficult that only affects reading. I don't want to minimise that.