What worked for you as a child doesn't work in your late 20s
The adult brain learns differently to the child brain. Before age ~25 people can learn fairly well by simple exposure, without having a structured study strategy. After that, the brain slows down and depends more on crystallized knowledge, becoming more resistant to learning[1]. But this doesn't mean you can't keep learning effectively, it just means you need a plan.
This post was inspired by a Huberman Lab podcast, which I found a bit vague. I'm summarizing the actionable lessons from there and throw out the rambling.
The first lesson from the podcasts is that adults need to focus in order to learn. You can't just expose yourself to background material; you need to pay attention, which normally means looking closely at the material and keeping your thoughts on it. No rambling podcast conversations, unless they spark a line of thought that you follow through. Prefer dense material and think about it.
In order to trigger learning you need to be comfortable with the feeling of frustration, and understand it as a signal that there's something more to learn. Once you're frustrated, you should push through it for at least 15 minutes, when you'll reach a brain state for about 1 hour when you can effectively learn whatever you're focusing on.
It explicitly doesn't work to pick up material for 5 minutes, get frustrated, stop, and come back in another hour. That is not an effective way for adults to learn!
You only have about 4 hours worth of good thinking time per day[2]. If you try to learn or do serious thinking beyond that it probably won't work; you should use that time for social activities instead or less-focused types of work where you can use already established skills.
I think it's also established that learning works better when it's appropriate for your chronotype.[3]
The podcast also implies that there might be a few hacks that you can use to speed up learning, or to get into the efficient learning mode more effectively. These are dubious but cheap to test.
Huberman cites a study that people learn faster when they need to learn to be able to get food[4]. You could extrapolate from that study to infer that maybe intermittent fasting is a good idea if you want to learn; being slightly hungry might help with learning. Anecdotally this seems to work for me; I feel sharper and more capable of thinking if I skip lunch one day a week. Overeating in the evening totally screws up my thinking for the next day.
Huberman also says that brain is hard-wired to learn balance above and beyond other skills. He talks about standing on one leg being a good way to release adrenaline and signal to the brain that it needs to learn. I infer: could standing on one leg for 15 minutes work equally well to pushing through frustration? I've not tested this one yet!