Hello Kern Carboni,
Thank you for reaching out.
I understand that you would like to know what role you need to assign to the developers in your Entra ID tenant to be able to manage your organization's Azure B2C tenant.
Please be informed that the Azure B2C tenant is a standalone tenant even when it was created from your main tenant.
To allow your developers access to this tenant, the users need to be invited to the tenant as a guest and be assigned necessary permission on the tenant depending on the Objects (for example applications) that they will be managing in the Azure B2C tenant.
NB: You can access and switch to the tenant because you created it. Your account is also external with the ExternalAzureAD identity in the tenant since it wasn't created within that tenant.
Let me know if further assistance is required.
Babafemi