sendAs
The Gmail setting that holds each sending address and its signature.
In Gmail, every address a person can send from is a “send-as” identity: their primary address plus any aliases. Each identity has its own display name, reply-to address and signature.
Google’s Gmail API exposes these identities as the sendAs resource. A signature tool with the right permission can update the signature field for each identity, which is exactly what a user does by hand in Gmail settings.
Because the signature is stored in Gmail itself, it appears in Gmail on the web and is added by Google’s servers for messages composed there. Mail does not have to pass through any third party.