Here you can find information about app architecture .
Jigit supports uni-directional and bi-directional integration between Jira and GitHub/GitLab.
Uses REST API of the Source Code platform (GitLab/GitHub/Gerrit), which means GitLab/GitHub is unaware of Jira, which decreases the security surface.
Jira runs a Jigit scheduled task every 2 minutes (configurable) using the configuration settings for each rule.
Using the REST API of GitLab or GitHub Jigit app retrieves commit and code reviews information and matches them with the Jira issues using the Jira issue key, like DEV-133, F1-454. The commit information is stored in Jira’s database (App specific tables) and associated with each issue when the keys are extracted from the git commit message or pull request title.
When somebody views an issue, they can see the commits and code reviews in the Jigit tab added by Jigit for that particular issue.
For bi-directional integration, the Jigit development panel can be used to be able to create new and link existing feature branches to an issue.