Bearer binding is done by PGW based on QCI and ARP.
Eg. in case of Video call there will be 2 bearers and it is PGW that decides that 2 bearers will be created based on the QCI and ARP sent by PCRF.
For each bearer usually, there would be 2 Charging-Rules. 1 will be for RTP flows and other will be for RTCP flows. Both the rules will have similar QCI and ARP.
I guess by session binding you mean Gx and Rx binding. If, that is the case then yes PCRF does the session binding between Gx and Rx. It is done by Framed-IP-Address.
However, it sometimes depends on the implementation also. A vendor can also do binding based on the combination of Subscriber-Id, Framed-IP-Address, Called-Station-Id.
Hope it helped.
Regards,
Peeyush Sharma