The whole brain switched on by Novelty, Interest, Challenge, Urgency thing is an oversimplified explanation for what dopamine does. It came originally from Dr. William Dodson, this is the article where he first talked about it and it then went viral from there, because a lot of people on ADHD social media related heavily to it:
https://www.additudemag.com/secrets-of-the-adhd-brain/
In this article he argues that neurotypical people are motivated by something being important (to the person), secondly by something being important (to someone who is important to the person) and finally by rewards and consequences, whereas people with ADHD are not motivated in this order at all, they are motivated by "ICNU" (which is the four things I mentioned above, I just find NICU easier to remember).
Honestly I think this is kind of an annoying explanation, because it's not really complete on either side. Neurotypical people aren't going about life purely being motivated by importance like some kind of perfectly efficient computer-robot people, even if they tend to take ADHD people's lack of motivation as a lack of thinking something is important. Neurotypical people are also excited by things which are novel, urgent, challenging etc, it's not like some really unusual motivation factor that only ADHD people have. ADHD is also motivated by reward and punishment, in fact possibly more so than most people, just the distance of the reward/punishment is more exaggerated - think of the marshmallow test. It reads more like pop psychology to me than a proper explanation for what is going on, and I dislike ADHD analogies that basically suggest "normal people find life totally easy and never have any problems". Because it's just not true, it leads people to go "Oh maybe I have ADHD because I also find it hard to be the person I want to be!" and it leads ADHD people to be resentful and assume that it's impossible to ever have any kind of level of functioning.
Dopamine deficiency has been discussed, dopamine is essentially "the motivation hormone", and we secrete it in response to things like completed tasks. In neurotypical people that builds a reward network where they know that completing tasks = sense of satisfaction and that enables motivation towards tasks even though those tasks may be boring or unstimulating in themselves. But people with ADHD do not get this response, partially we think because the dopamine is not produced in the correct amounts (like how some forms of depression are caused by insufficient production of serotonin, and SSRI antidepressants prevent the reuptake of serotonin, allowing it to stick around and do its job longer) and partially because performing tasks with ADHD uses up so much energy and brainpower (due to other neurotransmitter deficiencies) that often even when someone with ADHD achieves something massive, we don't feel a sense of elation and satisfaction, we just feel relieved. And then also people with ADHD often struggle to actually complete tasks fully, which is another reason that we don't get that kind of brain-training loop of complete boring task > get reward squirt of dopamine.
I'm losing track here. I need to go to bed. But essentially, what's happening is that interesting/novel things are causing you to secrete dopamine, which is why they are motivating (to everyone) - with the difference being that NT people expect to get dopamine from doing "important" things, plus have the ability to hold that important task in working memory for long enough to formulate a plan as to how to execute it, sometimes almost unconsciously. ADHD people struggle with executive function and therefore cannot hold as many items in working memory, and are less likely to be able to formulate a plan to do them without actively sitting down to do that e.g. with pen and paper.
Urgent things cause us to secrete adrenaline, which fits into noradrenaline (norepephrine) receptors and that's the other neurotransmitter that ADHD brains are short on and it helps with executive functions. You can use adrenaline as a noradrenaline substitute but not for very long because it's short acting and because it will give you an anxiety disorder if you keep relying on it all the time.
I have no clue where he's coming from with the challenging one because I don't experience that. If something is too challenging I tend to give up very quickly, I have very poor stamina or tolerance for frustration, and this is also common with ADHD.
Anyway if you want proper explanations for how ADHD brains are processing something in comparison to a neurotypical brain, listen to Russell Barkley.