Navigate to the Source tab on the sidebar. Select a specific file in the repository. Choose a file revision from the History menu and click the Contributors to open a drop-down list of all contributors who changed this file up to this revision.
In the list you can see contributor names, a number of lines they changed in this file and a number of their commits that modified the file. If a contributor is a Bitbucket Server user, clicking their name will take you to the contributor's Contributions graph, where you can check contributor's overall performance and all their commits and pull-requests made over the last year/month/week.
To see all repository contributors, scroll the list down to the end and click View all repository contributors.
Bitbucket Server also can show which lines were changed by a contributor. For that, click Blame and see the change log in the appeared sidebar on the left.