1. Why Lightroom 5.7 opens when I download photos from my card reader instead of Lightroom CC 6.3?
Most likely because LR5's Preferences is set to "Show Import when a memory card is detected" or your OS's Auto-Play (Mac equivalent) is set to launch LR5, while the same is not true of LR6.
2. Also How to move the Venice/Belgium photos from Lightroom 5.7 to Lightroom CC 6.3?
If you want to retain LR5 on your computer, open it and in Library select the Venice/Belgium folder. Then under File select "Export as catalog", putting the exported catalog in a convenient place. Close LR5 and open LR6 and select "Import from catalog", selecting the catalog exported from LR5. Delete the exported catalog after it has been successfully imported to LR6.
However, I can see little reason to retain LR5 if LR6 is working well. I would backup the LR5 catalog and then import the entire LR5 catalog (Lightroom 5 Catalog.lrcat) to LR6. Once you are satisfied that the entire catalog has been imported properly, you can uninstall LR5 (that will also solve the memory card problem) - but keep a record of your registration number and keep that backup copy of the catalog, if you think you might want to reinstall it in the future.