Styling products don’t generally cause damage although it will depend on their ingredients.
The cuticle layers that surround the hair shaft are like the scales on a fish and lie flat in one direction. When they’re undamaged, they reflect light very effectively so your hair looks shiny and is therefore healthy.
We all shed generally between 50 - 150 hairs a day naturally as part of the hair growth cycle and people with long hair tend to notice it more because their loose individual hairs are longer and more easily spotted. The rate at which your hair renews itself depends on the individual and some people will have individual hairs that are more than 7 years old on their scalp whilst others find that their hair stops growing and sheds much earlier.
Lots of things can damage the cuticle scales and cause them to break off. Imagine if you have hair that is 7 years old on your head. It will have been washed hundreds of times, combed and brushed, been exposed to sun and wind, possibly chlorine or sea water etc. That’s before you consider the chemicals you apply to wash and style it with and the styling with hairdryers and straightening tools. Now imagine hair that has also been chemically altered with colours and bleach.
One easy way to check basic hair health is the elasticity or wet stretch test.
A lot of damage can be done by silicone buildup, especially when using heat. Unfortunately, most styling products are packed with silicones. Many shampoos also contain silicone, so it becomes a viscous circle.