Considering the dumbbell overheadpress, benchpress and so on. I find that in all these exercises, an important point is to not go beyond ninty degree at the elbow. Why is this?
It’s fine to go past 90 degrees.
There is no reason to avoid degrees of elbow flexion beyond 90 degrees in pressing movements. Your elbows and shoulders were made to do that just fine. One thing to consider is that taking a joint through a greater range of motion means that you are doing more work in the same number of reps than a set with less range of motion.
There is something to it, but clinging to 90 degrees is nonsense.
Consider how muscles work: the myosin filaments glide in between actin filaments. Now, if muscles are stretched beyond a certain threshold, not all heads of the myosin can make contact with the connection points on the actin. The result is that beyond a certain length of the muscle, it cannot exert as much strength as it can when it is shorter.
That can become problematic when you train with max weights and go "too low" as it means the potential strength of your muscles may become lower. At some point, the muscles won't be up to the load anymore and you collapse at the lowest point although the muscles, as such, had more reps in the.. Therefore, it can cost training volume.
Another point is wrist angles: ideally, your index and middle finger knuckles should roughly stay in line with the ulna and radius bones of your forearm to load the wrist optimally. Grip width and full range of motion can lead to odd angles in the wrist which can become a problem with high load and volume.
Why is the 90 degree dogma nonsense, then?
Well, first of all, you don't train with max weight only. The structures don't have a problem with going below 90 degrees even with load as such. As a matter of fact, muscles (or rather: moving structures as a whole) tend to react better to training stimuli when going through their "full" range of motion.
The more important point, though, is that individual proportions can be very diverse: depending on how long your upper arm is, and how long the forearm compared to that, the leverage can be very different. Same for the wrist: If your grip width is adjusted to your proportions, you can easily go beyond 90 degrees flexion and still have your wrist aligned well. This means that while the tipping point may be at 90 degrees for some, it can be much more (or less) for others.
Therefore, you need to try out what works for you and your body. Nobody can give you magical formulas that tell you what to do.