If you have your arms at your sides, elbows should bend 90 degrees. Then just move your arms slightly forward and you'll end up somewhere around 95 degrees. Now you can rest your forearms on the desk. This won't save you from all kinds of RSI, but it might help your wrists, elbows and shoulder joints last a bit longer.
In either case, the most important thing is to keep your wrists in as straight and neutral position as possible, with your palms and wrists "floating" rather than resting on anything while actively typing. Having the wrists either flexed downward or extended upward is a really bad idea. Having the wrists turned out to the side isn't great either, but not as bad.
The keyboard should be positioned close enough to your body so that your shoulders can be relaxed with your upper arms hanging loosely. The laptop surface should be roughly parallel to your forearms, so if you have a high desk or table relative to your torso you will need to prop up the far side to tilt it up a bit.
For example, the monitor should be at eye level vertically but with laptop that's very hard to accomplish unless you position yourself in a reclined fashion to bring down your eye level closer to your lap - on a macbook you get wrist cuts like this.
One of the most important thing that makes a good ergonomic laptop is the ways it accomodates as many positions and setup as posible so your can rotate your working position to avoid excessive strain on one particular area. So when your back is tired you slouch down, when your wrists are tired you straighten up, when your eyes are tired you adjust the display brightness/theme etc.
When taken seriously it's totally possible to work safely even in poor conditions like outside or on a train but devices that completely ignore ergonomics just don't even give you the chance.
This is slightly misleading advice. The ideal place for the display has the top of the display at roughly eye level, or for a very large display maybe slightly above, which puts most of the display below eye level. Humans actually have great ability to look slightly downward for long periods of time while doing stuff with their hands, even while keeping their head held up straight, and indeed our eyes can more comfortably focus on close objects in the lower part of our field of view than straight ahead. What you don't want to do is slouch or bend your neck too much.
A laptop display attached to the keyboard usually isn't an ideal placement, but it's generally not too bad.
Welcome to "tech neck" - upper crossed syndrome, from looking slightly down.
You're inviting some surprising symptoms, not just neck and back pain, but things like numbness, tingling, or pain shooting down your arms. Really not fun.
Key posture correction seems to be pulling head back. Some physical therapy exercises can help as well.
https://deukspine.com/blog/tech-neck-forward-head-posture-tr...
In trying to picture this, I suppose there are certainly some stock photo models who'd feel the sharp edges:
google.com/images?q=person+using+laptop
I totally know what you mean about shifting positions. All the positions I've been in where I've felt the edges have been quite unergonomic, but perhaps not for everyone.
It's almost like y'all are different people...