STRETCHING affects your viscoelasticity - the exhibition of both elastic & viscous behavior, which with maintained stress can cause permanent deformation.
But it’s mostly short-lived. STRETCHING does not permanently affect a structure.
What does happen is that one becomes more FLEXIBLE by changing your STRETCH tolerance. What does that mean?
Well, FLEXIBILITY is governed by your Nervous System via your STRETCH Reflex (the way a muscle contracts in response to STRETCHING). It is a protective reflex which provides autonomic regulation of muscle length.
So, becoming more FLEXIBLE is about convincing your STRETCH Reflex to allow you to go into increased ranges. It is about convincing your Nervous System to change; to allow you to have access to an increased range of motion.