What's actually the difference between having ADHD and just being lazy/unmotivated?

The 'do you actually care and still can't do it' distinction is the one that clicked for me too. I'll forget to eat because I got absorbed in something for work I actually love, that's not indifference, that's a broken 'switch tasks' function, not laziness by any definition I've heard.

There's a real, meaningful difference, and it comes down to whether you can choose to do the thing when it matters, not just whether you find it boring. Laziness, as a concept, implies the ability is there and the person is choosing not to use it. ADHD is closer to the ability itself being unreliable, you can want something intensely, know exactly why it matters, have every intention of doing it, and still be unable to make yourself start, not because you don't care but because the part of your brain responsible for initiating non-interesting tasks isn't reliably cooperating. The tell that convinced me this wasn't just laziness in my own case: I'd miss deadlines on things I genuinely cared about and would be devastated about missing, not things I was indifferent to. A lazy person, by the colloquial definition, usually doesn't feel much distress about not doing the thing. What's common in ADHD is the opposite, real distress and self-criticism about not being able to do something you actually wanted to do, over and over, in a pattern that doesn't respond to just 'trying harder' the way garden-variety procrastination usually does. It's also not really about difficulty level, plenty of ADHD people struggle with easy, boring tasks while managing genuinely hard, interesting ones without issue, which is the opposite of what you'd expect from someone just avoiding effort in general. Doesn't mean laziness as an everyday concept doesn't exist at all, just that a diagnosable, impairing pattern of not being able to start or sustain things you actually value is a different animal.