mikseogie gwailgwa uyureul neohgo garaseo seumudireul mandeureoyo.

In this sentence, -에 marks the target or destination (“into the blender”). 믹서기에 과일을 넣다 literally means “put fruit into the blender.”
If you used -에서, it would emphasize the place where an action occurs (“I’m blending at the blender”), not the location you’re inserting something into.

When listing multiple objects connected by 과/와, you only need to attach the object particle 를/을 to the final noun.
So 과일과 우유를 = “fruit and milk (as one combined object).”
You could say 과일을 우유를 for emphasis, but it’s not necessary.

-고 is a verb connector that strings actions together in sequence, similar to the English “and.”
Here, 넣고 means “put (them) in, and then…” It simply lists the first action before the next verb.