PduR is not receiving/sending signals directly, it only handles I-PDUs. It is the COM module's responsibility to wrap/unwrap between signals/PDUs.
As Uwe mentioned in his answer it depends on the ECU Config which PDU will be routed towards which physical interface. Think of PDU Router runtime as a static routing table, built upon the configuration values.
I circled for you with orange the exit points towards lower, bus-dependent layers. PduR treats multiplexed, secured or container PDUs differently, sending them not "down" but "up" on the diagram below.

You can figure out where exactly an I-PDU will be forwarded by searching for config elements of type PduRRoutingPath
. You will find them under the 'PduR' config module. Depending on the exact AUTOSAR version you will find at least two reference parameters called like 'dest' and 'source' pointing to PDU descriptions.
- One of the referenced PDUs will have another referee in a
configuration Module with a name like 'CanIf' or 'FrIf'. Now you know
which interface its heading to or coming from.
- If not, then your signal can be in a container or secured I-PDU. Then you need some
further digging in the config, as the
PduRRoutingPath
you find will
have its other reference pointing to an I-PDU defined in SecOC or IpduM configuration Modules.
I-PDUs of the latter types are not leaving/entering the ECU in their plain format. The router routes such I-PDUs "up" to corresponding modules for processing.
Such wrappings can even be chained: a signal PDU can be multiplexed and encoded at the same time. In this case, a transmit signal's data would do multiple in and outs on the top of the PDU Router, before the payload actually leaves thru one of the bottom exits I circled.