My over-simplified explanation: the graphql server will map a field on a request to a resolver. The resolver can execute whatever code it needs to return the value, up to and including calls into libgit.
So asking for the title of a PR might just be an extra column selected on a DB query. But calculating mergability status of that PR might be something else entirely.
So asking for the title of a PR might just be an extra column selected on a DB query. But calculating mergability status of that PR might be something else entirely.