I'm building a process that uses a REST call to fetch external data; the external server requires to authenticate using Certs. Is this possible in APS? So far I have used REST calls using basic authentication but never certs.
I'm assuming with "certs" you mean certificate-based authentications using a digital certificate.
If my assumption is correct, APS uses the basic authentication only at API level and more advanced authentication mechanisms through the AIMS (Alfresco Identity Management System based on Keycloak). Please note that AIMS is supported by APS only since 1.9 or 1.10 (I don't remember now).
In every case, I don't think that any specific digital certificate is supported by keycloak (so AIMS) and I'm afraid you will need to create something custom for this use case.
For more detailed info on APS, you can raise a support ticket (assuming you are an Alfresco APS customer).