Btrfs has data and metadata chunks, all the data you copy there will use raid0, metadata (and system) is where the filesystem information is stored, only that part is raid1, if you for a example get a bad sector where metadata is stored you can lose the complete filesystem, with raid1 metadata there's still another copy if that happens, and again metadata takes so little space compared to data that it doesn't make sense to use raid0 for it, but you can if want.