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This appears to be the doc on Google Managed Certificates ...
https://cloud.google.com/load-balancing/docs/ssl-certificates/google-managed-certs
According to the doc, "... and Google renews the certificates automatically." Is this sufficient for the expiry date question? I sense that Google ensures that the certificates are renewed before they expire.
Google gives you two certificate stories ... one where Google manages your certificates (see above link) and one where you manage the certificates (self managed certificates) ...
https://cloud.google.com/load-balancing/docs/ssl-certificates/self-managed-certs
In the later, you are responsible for obtaining the certificates and then following Google's recipes for installation and use.
It is my understanding that both the self managed and google managed certificates work only in conjunction with Google Cloud's load balancer technologies. You can (in principle) use other Google Cloud services in conjunction with these certificates ... however, you would front end those services with Google Cloud's Load Balancers. For many of Google's services, they are SSL protected with certificates owned by Google ... for example if you try and target Cloud Run or Cloud Functions through HTTPS you are using certificates and everything will be secure ... they will just be Google owned certificates.
While Google is a Certificate Authority (see ... https://pki.goog/) ... consumers can't get certificates from Google for their own user (see https://pki.goog/faq/#faq-29) Instead, the certificates are used/supplied by Google products (eg. Google Managed Certificates). In your question, the answer is then that you will indeed need to procure SSL certificates from "outside Google" and bring them to Google for usage. This is the Self Managed Certs story.