It's usually a place where you're not bombarding your brain with stimulation all the time, so your brain tries to use the downtime to work with all the information you've been putting into it.

This is actually a big part of learning anything, called diffused learning. Think of how you suddenly get something after a period of rest or not doing anything, some time after you've initially focused on a thing for some time - your brain has actually been using the downtime to structure the data and make better sense of it.

This is also why a lot of people that know how a human brain works suggest mediation and walks, especially without listening to music or podcasts, as well as spending little to no time reading, watching or stimulating your brain in any other way before bed. It needs that time, it's crucial for development. Journaling helps here, too, because it's both reflection and somewhat of a downtime.

Doing chores and not listening to anything counts, too.

But it's all easier said than done in this age of constant hunt for one's attention so they spend more time on your app, giving you more data to sell and more metrics to make the line go up (gotta keep the investors and stakeholders happy, can't afford to not show constant growth).