I have a serious question in mind. After submitting a project, are we allowed to modify it or re-submit a better version? I’ve noticed that the submission field doesn’t deactivate after I submit my first 2 projects.
Yep! If you would like to improve as you learn new tricks and best practices you are more than welcome to resubmit.
Which version will be considered for plagiarism checking? I copy-pasted the example link into the assignment link field to see what would happen (very stupid in hindsight) and now am scared about my certificate being cancelled. Can I resubmit with my own version, or am I screwed?
You can resubmit any of the projects the same way you submitted originally. The latest submission is the one that gets stored.
Just make sure you resubmit that project before you claim the certificate, to be safe.