Simply we have built the app and uploaded to Crashlytics. In case when issue arise we try to check what went wrong but we can't because:
How can we upload missing dsym files? Where can we get it from?
OR
What should we change in project to upload dsyms automatically.
dSYMs files for each archive are found locally on your build machine at the directory /Library/Developer/Xcode/Archives
Right click on your xcarchive file then Show Package Content, you will find your dSYMs directory there, you will have to zip them and upload them to Fabric manually.
You can also get the dSYMs for a specific build from your AppStoreConnect activity page.
By default Fabric dSYMs are uploaded automatically after the archive through the Fabric /run script added to Build Phases in the beginning of Fabric integration.
Though it might fail sometimes for different reasons. Like having the Bitcode enabled
You can also use different tools for build automation like Fastlane which provides an upload dSYMs command.