The developer would most likely implement a Root CA in the autonomous vehicle's software. A Root CA is the top-level authority in a PKI that issues and validates certificates for subordinate CAs or end entities. A Root CA can be self-signed and embedded in the vehicle's software, which would reduce the need for external communication and verification. A Root CA would also enable the vehicle to use digital signatures and encryption for secure communication with other vehicles or infrastructure. Verified Reference:
https://cse.iitkgp.ac.in/~abhij/publications/PKI++.pdf
https://www.digicert.com/blog/connected-cars-need-security-use-pki
https://ieeexplore.ieee.org/document/9822667/