How is the trigger pulse of the 555 timers generated?
There are MANY block diagrams and descriptions of the internal workings of a 555 timer on web. Looking at some of these will allow you to easily answer your own questions. Having people provide another version of what is already so widely available is pointless.

You'll find a zillion diagrams with linked web pages here



Operation of one mode explained with block diagram drawn in terms of standard circuit elements here

Diagram from that page:





And another here

Related block diagram: