C: Obtain the root CA certificate (step 4 in the picture below)
D: From KV1, create a certificate signing request (CSR) (step 2 in the picture below) Note:
Creating a certificate with a CA not partnered with Key Vault
This method allows working with other CAs than Key Vault's partnered providers, meaning your organization can work with a CA of its choice.

The following step descriptions correspond to the green lettered steps in the preceding diagram.
In the diagram above, your application is creating a certificate, which internally begins by creating a key in your key vault.
Key Vault returns to your application a Certificate Signing Request (CSR).
Your application passes the CSR to your chosen CA.
Your chosen CA responds with an X509 Certificate.
Your application completes the new certificate creation with a merger of the X509 Certificate from your CA.
Reference:
https://docs.microsoft.com/en-us/azure/key-vault/certificates/certificate-scenarios