Where does the Jira 'mentioned on' issue links come from?


I’m looking for the place where I can get the relationship between a Jira issue and a Gitlab commit/merge.

Both Jira and Gitlab are local deployments (gitlab is running inside a docker container).
I’m talking about the links that appear when you configure the Jira service on Gitlab:

Is there some sort of API endpoint on either Gitlab or JIra that I can get these things from? Alternatively is there a table in the database that I can look at (again in either product)?
I’ve asked on the Jira forums and they’re not quite sure either.