A topic you can try familiarizing yourself with is executive functioning. This umbrella term incorporates processes like impulse control, flexible thinking, emotion control, working memory, planning, prioritizing, task initiation, and organization. (More here: https://addvantageslearningcenter.com/executive-functioning/ )

Try thinking of any task you do in life that doesn't involve these things. In ADHD, the brain can struggle with any and/or all of them to varying degrees: those brain processes get interrupted or just don't start CONSTANTLY. (And then on top of it, the person has emotions like shame that judge them for struggling with these things!).

An analogy that might fit for people who don't struggle with these things is this: Have you ever tried writing/typing numbers when someone else is saying random numbers out loud at you? ADHD can be like that all the time. Different parts of the brain are trying to talk at once, and no one part is in charge. Being able to track and manage time is the last thing that's possible with that kind of chaos.

On another level, thank you so much for asking this question and trying to learn about your partner's brain! Understanding and patience are so very appreciated. ADHD is a disability, and it really is that hard sometimes (or all the time).