Technically speaking, when you upload a comic to a certain platform, you are not giving away your copyrights. What you give them is the license of your copyright. License of copyright can either be but not limited to: licensed to host, publish or use as commercial products. You can restrict what a license holder can use your work with.
Of course, you can give away your full copyright by signing a contract. With this, you basically transfer all your rights to another party and you, yourself ends up not having any rights to it other than your name as the original creator.
So to steal a copyright, you must be convinced to sign a contract with clauses that says you give away all rights to work, without you knowing. This is main reason why you must always read everything within the contract and don't just skim through.
Edit: I misunderstood the first question... blame my brain for not being awake yet. Technically, copyright exists the moment the work is created. Timestamp your work. And unless you really want to be secure, only then you apply for the copyright thing(?). Never really did it so I'm not sure.