I watched a video on youtube about double slit and it said that when we try to observe electrons it starts to act like a particle but what are these observers that they use? they're shooting one electron at a time so how can they detect them while they're moving in front of the observer?
An observer is you or someone looking at the electron. It can also be a detector or camera taking pictures etc. it is wrong to think that the mind of a conscious observer will cause an observer effect. The thing is if you look at an electron as it's traveling through the slits you will affect it's a trajectory because you physically interfered with it. In order to see the electron photons need to interact with it and they will affect its path causing the overall interference pattern to be disturbed.