Hello,
it looks like theres an animation that holds the weapon that the sight is right in the center. I really want to implement that in my project but i dont know how to realize that, if someone has ideas, I really would appreciate that.
FP aim down sight Pubg: https://www.youtube.com/watch?v=4v-xwUHCcbU (you can see it at the beginning )
So you pretty much hit the nail on the head. It is just a fancy graph that goes from a hip shot idle up to a aim down barrel idle. The zooming in is then just a lowering of the player characters camera FOV.
If you are looking for tutorials or tips on how to go about doing this, then for the animations there should be some generic tutorials on Youtube for creating animations for first person weapons. Something like this UnevenElefant5's First Person Animating Tutorial (1 of 4) - YouTube
Then you going to need to import those animations and set up an anim graph that you can then use to swap between the states.
Tutorial on Anim Graphs BP 3rd Person Game: Building the AnimGraph | 10 | v4.8 Tutorial Series | Unreal Engine - YouTube
Then finally for the character BP, you will need to trigger of the state change and during that get the camera and linear interpolate from the default FOV to the zoomed FOV
Not really something that can be done easily without a bunch of work. Unless you sort of ignore the animated blend in / out and just toggle between being the two states and just jump the FOV around. 
Can it also be made by blueprints/c++?