but they're not 90º arched either. I see I curl them a lot in a kind of rest position but I tend to flatten them for the top row keys and curl them for the lower row.
At resting position they're not curled
Curling fingers require shortening of the tendons and can only be achieve by consciously maintaining the tendons in that position. Naturally the resting fingers with the tendons relaxed are like the one in the first pic. My problem is that keeping fingers even slightly curved requires an effort, tension and tendon/muscle use wich is not only unnecessary but create co-contraction by causing the movements to overlap the already cronically tensed tendons/muscles.
What you say about keeping them flat for the higher row and curved for the lower row proves my point about distances
A lot of pianists sit
too close at the keyboard (either piano or computer)
This causes a lot of problems including non-free movements, raised elbows and chronically raised shoulders. Another problem is that it is comes natural in a too-close position to curve the fingers even if that implies chronic tension
What happens is that when you are playing or typing far you can keep the finger in their naturally arched semi-flat position (hence long fingers) because there's enough room for the hand to move backward and allow the fingers to elongate
When you're playing or typing in the lower rows (closer to the body) the hands can't move back to allow the fingers to elongate IF this moves the elbows back toward the side of the body and LOCK them at the sides. So what one instinctively does is keeping the hand in the same position just shorting the distance between the playing part of the fingers and the hand (hence curling hence shortening)
For example let's say you have to press F8 with long arched semi-flat fingers
This mean the palm will be above the lower part of the keyboard near the space bar
Now let's say you have to play the J
You can do two things
1) keeping the hand in the same position but curling the fingers till you can reach the J
2) keeping the fingers in the same natural position but moving the hand backward till you can reach the J
The 1st one is IMO less natural, desirable and effective and is probably harmful and it's what come natural when we sit too close so that moving backward would automatically lock the elbows at the sides or ever worse move them behind the body
The 2nd one is what we should do and what would come natural to the instinct of the body AS LONG AS we sit at the correct position that allows the moving backward of the hand without locking the elbows at the sides
The correct position IMO at the computer keyboard and the piano is the length of your forearm minus the hand between you and your upper body
If you're doing this right your elbows should easily (if needed) go in front of your torso and without the side of your body stopping them

I guess the main thing is to let the 'cushion' of the finger hit the key (in the piano) and not the tip where your nail would hit it instead.
Exactly but if you do this (hitting with the cushions) while keeping your fingers high you will end up with a collapsed wrist, high knuckles, straight fingers with last phalange totally bend. Such position is even worse of the high fingers position when you hit the keys with the naily tips
The only way to naturally hit with the cushions (it seems to me) is to assume the position I showed in the first drawn pic (slightly naturally arched, all fingers joint aligned, rather flat position, long fingers)
Also that position is the only one that you would maintain without any kind of effort or strain. Your fingers would be in that positions even if you were sleeping or had lost consciousness wereas all other fingers position require a chronic conscious tension of the tendon/muscles to maintain the position and would be naturally lost (i.e. the finger would naturally return to the slightly-arched semi-flat position) as soon as the effort and control is lost or losened a bit