Here you can find information about app architecture .
Jigit provides uni-directional integration between Jira and GitHub/GitLab using REST API of Source Code platform (GitLab/GitHub → Jira), which means GitLab/GitHub is not aware of Jira.
Jira runs a Jigit scheduled task every 2 minutes using the configuration settings for each rule.
Using REST API of GitLab or GitHub Jigit Jira App retrieves commit information and extracts there all issue keys, like DEV-133, F1-454. When the keys are extracted, the commit information is stored in the Jira’s database (App specific table) and associated with each issue.
When somebody views an issue, they can see the commits in the tab added by Jira Jigit App for that particular issue.