ADHD at its core is a dopamine disorder. Effort without dopamine is extremely difficult in anyone, and ADHD makes it tough for these dopamine mechanisms to get going. Once that dopamine effect does get going, you're off to the races. That's the ADHD Hyperfocus effect you see.
But when you add in the modern quick dopamine fixes we've surrounded ourselves with—media, social media, processed foods, pornography, etc.—you've built a situation where ADHD individuals are prioritizing those activities over productive ones. This leads to a downward spiral: less productive action means fewer opportunities for future productive actions.
Lock into that mode for weeks at a time and you've got both situationally-supported and biochemical depression.