So I keep asking myself, wether there is a correlation between the rank I have and the reward I get. is it the same when I level up to lvl 2 and when I lvl up to lvl 25?
There is no correlation between your matchmaking rank or profile rank and the quality of weapon drops you receive.
Weapon drops are purely randomized in a way that makes sense: the lowest-quality weapon skins (industrial grade) are the most common while the highest-quality weapon skins (covert) are the rarest. The chances of getting a covert skin in any match are less than 0.1%.
Note: This is separate from Operation-induced drops. In Operations, such as Bloodhound, ranking up gets you a Bloodhound drop. These will automatically be at least slightly better than any non-Operation drop.