When preparing my app for release I take these steps to export, upload, and get the dSYM's:
- In terminal I run 'flutter build ios --release'
- Open the iOS project in xCode and switch the device to 'Generic iOS Device'
- From the top menu Product>Archive
- Once finished the Organizer window will show your archived build. You can also manually open this window from Window>Organizer
- Choose the build you want to upload to iTunes Connect and hit Distribute App and follow the process
- After upload is complete right click on the build in the organizer window and click 'Show in Finder'
- You should see an archive file in finder, right-click it and click 'Show Package Contents'.
- Inside there should be a folder called dSYM that you can zip and send wherever you need