Yes, the iPhone SE looks and feels strikingly similar to the iPhone 8. But can you swap a few of their internals? Well, a teardown by iFixit suggests that you can do that for a few parts, but not key components such as the processor or battery.
What about the battery? You can’t. That’s because even though they look identical, the logic board connector of iPhone SE’s battery differs from that on the iPhone 8. Also, the Touch ID fingerprint sensor can not be interchanged, and you have to pay Apple for a replacement if you damage the one on your iPhone SE.