I think mostly about the player when I play. Stage knowledge is subconscious (I've played on Battlefield enough times to know that my U-Tilt and U-smash are my two ground moves that hit people on the platforms above me, but if an opponent has my banana U-smash can make me trip), and spacing you have to be aware of but it's not something I actively think about. I just think of reading the opponent and zoning most.
For example, I just played a (Wi-Fi) match a couple hours ago and I was completely focused, and this was my mindset in the first stock — just figuring him out.
I'm Diddy. He's Captain Falcon. We're playing on Battlefield.
The match starts, I pluck a banana, and he approaches but a bit hesitantly, I shield a Dash Attack with a banana in my hand, which gives me the idea that he doesn't know what attacks are safe on shield against Diddy (meaning I'm more likely to shield in the match) and maybe doesn't know how to use bananas.
A couple seconds later we're on opposite ends of the screen, I just shoot peanuts. He approaches me with awkward full hop D-airs and goes over the top of the BF platforms to get to me. This gives a greater hint that he doesn't know what to do against bananas, is afraid of the bananas, and therefore probably doesn't know how to use them. When most players don't know how to use bananas, they throw them back at Diddy within the first few seconds of getting them.
So the match continues, a few seconds later he gets a banana in his hand. I land and shield, and my prediction was right; he runs and throws it at me, so I SH Z-catch it and soon drop it afterwards as he's running towards me.
This gives me the idea that I don't have to play defensive; I can probably rush him with bananas and he won't know what to do. I try this and he uses pretty good defensive options, like shielding then rolling away, or spotdodging, etc.
This gives me the idea that I'll be a bit more pseudo-aggressive. I'll zone him at midrange (a range where Diddy excels) and either play defensive to bait attacks, or throw out a banana/run around a bit in his range to bait a defensive reaction I can punish with the other. It's almost footsies. This works extraordinarily well, and I dish out at least twice the damage I receive.
Now he's at high percents. With how I'm playing, it's difficult to punish something with a banana trip and still be in range to kill with D-smash or F-smash, because I'd have to be a bit closer (which I didn't want to be, as mid-range was working well). Another common reaction with bananaphobiac players is rolling away whenever tripped. A lot of players, even ones who know how to react to bananas, still commit to one reaction, so it's worth paying attention to. Through the stock from early percents, I payed attention to how he rolled when I was out of range (without commiting to a kill move as I didn't want to whiff, and then he'd know what I'm doing and make it difficult for me). Sure enough, he rolled back every time except one where he tripped very close to the ledge (he rolled towards the center). Common reaction. When it was time for kill percents, I got a trip at mid-range and dashed past his trip. He rolled back and I d-smashed him.
Through that one stock I learned:
-He threw bananas back at me nearly every time he got them
-He took awkward full-hop approaches towards me, often resulting in him landing on a platform which was punishable with U-tilt or U-smash
-He usually rolled back after being tripped
-He reacted well defensively, so I tried to bait attacks to punish (C. Falcon doesn't have the safest approach options either)
-He didn't grab much, instead opting to attack my shield
-He'd sometimes do Dash Attack or Falcon Kick when he was close to me (also punishable)
-I should zone at midrange
That was a much more simple example. Against people who know the Diddy match-up pretty well, or are at least comfortable with bananas, there's a lot more potential situations I have to pay attention to and remember reactions of, as well as remember all my mix-ups, gimmicks, and situational tricks (diddy has a ton lol). But that's the sort of things I usually think of when I'm playing.
I can't say I'm successful all the time though. Because I depend so much on reading the opponent, people/characters with a lot of mix-ups, and people who play unpredictable usually trash me. I think of what I do second, so sometimes when I'm feeling pressured I play within the same habits and am predictable myself. Those are things I need to work on, but I know where my mental strengths and weaknesses lie, and in my opinion Brawl is a very mental game.
I also glossed over and forgot a lot of things that I thought during the match in my example, but that's just a general idea of what I think, hopefully it helps you a bit or gives you some ideas.