PPP over e for vdsl or a for adsl is a bt wholesale choice. Other LLU providers like sky don't use ppp.
As used by BTW it's only intended to be one device per line.
It's nothing to do with any sort of local area access.
How it's put over the wire (or maybe fibre) is very complicated at the detailed level of how *dsl or gpon works - but less complicated thinking (for vdsl) of eth frames go in one end and come out of the other
A pppoe frame is just an ethernet frame with it's own ethertype the ppp header is in the payload leaving 8 bytes less for the (usually) IP packet.
The broadcast part - I don't know the detail of how openreach do things at that level. It's possibly along the lines of TR 101 (search for it, also there are BT sins easily available that have other info in them).