How to use the tickets which are booked online?
Where are you booking the tickets from? The official site? Klook? Other 3rd party discount sites?
I am booking tickets from the official site
Print ticket and scan at the gate.
you can use them direct from your email, scan the ticket barcode. you can save the barcodes as a screen shot if you have no access to internet, but iirc there is free wifi at the Zoo.