How does the l2_switch.p4 implement the learning process?

[Kidel]

Hello.
According to P4 manual, the generate_digest action sends data to something according to its firt argument.

In the original l2_switch.p4 example on the BMV2 repository there is a C program that gets the digest and adds entries to the tables based on port and mac address.

How is that achieved here on PISCES? I can't find any program that receives the digest and implements some learning logic.

Thank you.