How is the design (main components) & working of a PABX phone system?

Question

also known as private branch exhange(EPBX)

Answer 1

The design is so complex that it wouldn't fit here (and I have to mow my lawn before dark....!)
The main component, the one that connects the caller to other PABXs and finally to the recipient, has changed over the past 40 years. First it was a 2-dimensional stepper relay, then followed by crossbar relays (Siemens called them ESD relays), shortly followed by a reed relay matrix, and nowadays it is semiconductors arranged in a matrix.