Bitcode is an intermediate representation of a compiled program. Apps you upload to App Store Connect that contain bitcode will be compiled and linked on the App Store. Including bitcode will allow Apple to re-optimize your app binary in the future without the need to submit a new version of your app to the App Store.
Xcode hides symbols generated during build time by default, so they are not readable by Apple. Only if you choose to include symbols when uploading your app to App Store Connect would the symbols be sent to Apple. You must include symbols to receive crash reports from Apple. After you distribute your app using App Store Connect, you can download the dSYMs file for the build.
For iOS apps, bitcode is the default, but optional. For watchOS and tvOS apps, bitcode is required. If you provide bitcode, all apps and frameworks in the app bundle (all targets in the project) need to include bitcode.
Is bitcode essentially same thing as IR? After a bit of digging, this appears to be true:
What is commonly known as the LLVM bitcode file format (also, sometimes anachronistically known as bytecode) is actually two things: a bitstream container format and an encoding of LLVM IR into the container format.