Depends on the thing.
Super high level, ADHD is an issue with the reward system of the brain failing to deliver reward when it's supposed to. Your brain is supposed to try to find a new task when it's not getting it's reward anymore; it's how that frontal cortex problem solving engine gets driven around by all the parts that handle motivation, wants and desires.
Sometimes no reward is being given, so you keep slipping off to a different task, and sometimes too much reward is being given and so you stay on a task way too long.
And, to be clear: these are not huge rewards we're talking about like a wave of pleasure or noticable feeling, just the baseline steering signals.

Sometimes the task you need to do provides no "normal" reward but neither does what you're doing right now, so your problem solver sees no reason to switch. Sometimes a nudge can help because fulfilling a request or suggestion can come with some reward, or at least you're just swapping out neutral tasks with some minor effort.

Sometimes the task is unpleasant to some minor degree, so not only is the reward not there, it's also a punishment. Or the thing you're currently doing is providing some degree of reward.
In either case, switching means actively going against everything your problem solver uses to decide what to do. Needless to say, that's really hard, and being nudged often feels more like being nagged, or like they're upset with you, because your problem solver (also known as your conscious self) knows this is all going on, but knowing how the engine is working doesn't make it work differently.
So you've been sitting there trying to push a granite block up a hill for an hour, and then someone comes up and starts pushing on your back. They haven't removed the part that made it hard, but they added something uncomfortable to your current situation.

Before I got on medication following my diagnosis, me and my partner handled it by just being really cognizant of what our mental states are, and communicating clearly. "You asked me to remind you", "I need to do it, but I'm stuck", and effectively asking for permission before annoying someone to the point where the current blocker is less desirable than doing the thing. Requires a lot of trust and good communication though.

It's difficult to describe subjective feelings, but what can sometimes look like "sitting on the couch watching short YouTube videos about sheep dogs instead of brushing your teeth and going to bed" is actually: sitting on the couch bored out of your mind and desperately wanting to go to bed, but the sheepdogs are providing short bursts of novelty and cute. Removing your lap blanket provides no joy and makes you cold. Standing up provides no joy and makes you less comfortable. Walking to the bathroom provides no joy and now you're in the dark bathroom. Brushing your teeth provides no joy, tastes bad, and is intensely boring. Walking to the bedroom provides no joy. Getting into bed and snuggling up provides joy.
Summed up: sheep dogs provide continuous minor joy, and only costs the physical misery of staying awake, the confused guilt of paralysis, and the promise of future misery. Going to bed is a promise of some joy, but it comes with a bunch of steps that are at best neutral and often entail anti-joy. It just doesn't add up. Other people get a tiny hit of joy from each substep, which is why they can say "I'm done looking at sheepdogs, I'm going to bed" and then just magically do it.

"Before you go to bed, you need to slowly press your bare foot into this fresh dog poop, toes spread of course" isn't often made better by someone saying "it's not that bad, come on, you can do it, I believe in you, then you can get some rest for once".