Your GoPro video is compressed using a scheme (probably HEVC) called “lossy” because information is discarded. It is OK to discard the information because it is too difficult for most human eyes to see. It is NOT RAW as some people call it. If it were uncompressed, it would be much larger than 6.6 GB. Each frame of 1080p video is 0.003 GB. If you are doing 30 fps, that means one second is 0.087 GB. If the video is 5 minutes, then it becomes over 26 GB! There is no way the camera could write to its SD card fast enough and would too quickly run out of storage if it were uncompressed or even “lossless.”
When a video file is decoded, transcoded, or edited it is converted to uncompressed temporarily in memory and then recompressed using whatever method you choose (see Export > Presets in Shotcut). It might not seem like that is possible for Shotcut to do when your computer only has, for example, 32 GB, of RAM that it must share. That is because it does not decompress the entire file into memory but only several frames at time. Finally, when you choose to export using a lossless method, it is still compressed but in a manner that does not discard information. Rather, lossless compression is somewhat like a zip file’s compression; a zip file cannot discard information, right? So, this is how it becomes much less than uncompressed but still much more than lossy compression. Lossy compression technologies are amazing modern marvels of efficiency and have helped facilitated our mobile and wireless revolutions.
I hope that helps, and if you want to learn more you should do a web search on “video compression 101” as there are other explanations that were better planned and prepared than ad hoc discussion replies.