Big Atlas of little texture, how UE4 read it ?

Hi !
I have technical question,(and I don’t really know how to find it on google).
How UE4 read the texture applied on an object. Does the engine read all the texture pixels and apply it if it’s in an UV “chunk” ? Or is there another method more optimized ?
Because I want to make a big Atlas of little texture for a mobile app, to reduce draw calls. But if the GPU read every pixel of a 2K Atlas when the object only need 64 pixels of it, it’s a big waste of GPU time I think ?
So if anybody have technical explenation of what’s the best technic.