You know the small pin like that sticks out of the tyre sidewalls, that one is the tyre indicator right?

Am wondering how it works, how does it get shorter as tyre wears out? It gets shorter based on number of years or based on tyre wear? How does it work?

What small pin?? Tire wear indicator is not like a small pin. It's like a bar of rubber within the tread pattern, going across the width of the tire. Once your pattern is level/equal height to that bar, then you already reach your max wear and should change the tire.
Another indicator on the sidewall of the tire, the shape of a small triangle just at the edge of the tread pattern. Once your tread reach that triangle, it also means that you reach max wear and should change.

I though the side wall near the edge got like hair like thing sticking out, that is not indicator? I see new tyre that thing quite long.

Can show picture?

How the sidewall triagle wear off since it's not under friction? Is it more of a date indication rather than wear indication?

Ooo that rubber hair thingy! That one is just due to manufacturing process, not wear indicator la!
Well I don't have picture now. It's quite easy, just look around the side of your tire on the edge of the tread pattern, there'll be a small triangle. It's not suppose to be worn out, but once the tread wear reach the point of that triangle, then you reach the max wear already.

in addition to the triangle, can look into the grooves of your tire, there should be some "bumps", those are indicators too

This is the pictures

Btw if you're using Goodyear, the triangle is replaced with the GY logo.
If I remember correctly for Michelin it's also replaced by the Michelin Man logo.

The triangle is to show u the location of tyre wear indicator..it is not an indicator.

if you dont have the tyre wear indicator, u can also use a 10 cent coin to measure. Just put it into the tyre groove, if the thread is not able to touch the edge of the leaf in the 10 cent coin, your tyre botak already.