I am too fond of brain dumping my ideas everywhere from my notebooks to my bear notes to my scrivener pages to my ms onenotes to my milanote cards. Basically, it's everywhere. [just don't copy my mess ]
Anyways, brain dumping those little details into one place helps a lot so you can have full access to one scroll of ideas whether it's a random one, a research on the plant kingdom, an existing power list that you've made for your characters, etc. If ever you feel that your plot is missing some sort of action like strategies on how to escape from a mountain pass, you should first recall your "brain dump folder" before you search unnecessarily on the internet. Who knows, maybe you'll find an idea about parkour in that folder. So yeah, it helps.
With this part you're talking about, then you must be referring to the atmosphere or ambience of the settings. This stuff is like a bane of my existence because this calls for effective narration. If you want your readers to feel what your MC experiences being in that certain place, your five human senses come into place. Those little details fall under those sense but you have to selectively choose those details that directly affect your MC. He has human feelings after all so we should relate to him cuz we're humans.
From my "brain dump files", maybe this link can help you because it has examples of a town's ambience.
Hope you can find little details that can help your MC's experience.
Hahaha! You don't have to "shamelessly promote" your novel to me because that's the very first novel I subscribed to in Tapas. Well, you don't have imagine me laughing maniacally...
Speaking of which, your tag says #evil mc but your blurb says your mc is a villain. Just be clear on one thing about your MC: a villain or an anti-hero? TY.