Can you determine an action reaction pair when tension acts on a string attached to a pulley?

I have been learning about tension and was doing some reading on trying to explain these forces at the intermolecular level and later also trying to see how it fits in view of Newton’s third law- I did come to certain conclusions but I am not sure if I am understanding them correctly.

As much as I have understood, I gather that on applying a force on either side of a string the molecules in the string move away from their mean positions and that tension develops as an internal force to try to bring the molecules back to their mean position. Is that a correct explanation?

WRT Newton’s third law of motion, consider a block attached to a pulley. When we draw free body diagrams for such systems, we usually indicate tension in two directions on the string. Am I right if I understood that because the block pulls on the string, tension develops in the string as a reaction force – acting on the block(but then how will tension restore the molecules in the rope back to their mean positions ?) and in the same diagram since the pulley exerts a force on the string and a reaction force develops which is the tension in the opposite direction.
But as action and reaction forces should be equal, according to my explanation the same string (let’s say massless) will be experiencing different tensions?

What have I understood wrong?

Thank you.