It depends. Vince McMahon makes you sign these insane contracts where they own your entire gimmick and ring name. So you cannot even use your ring name in your social media or appearances outside of the WWE. Imagine if Conor was not allowed to call himself 'Notorious' or Silva 'Spider' because UFC owned those nicknames. WWE also does not allow social media like Twitch streaming where you can make huge money in donations which is a major point of contention among WWE stars. They also sign you into contracts where you basically make appearances or endorsements over anything even if you do not use the WWE name.
People think that the Reebok deal was terrible? The WWE has things that are far worse. Also WWE has always been notoriously cheap with things like hotel and airfare compensation or daily allowance for food or travel. But UFC is also often terrible in that category as well. Currently the WWE wants it so that anyone under contract with them that makes money through social media like Twitch or Cameo must do so through the WWE and split the money with the WWE.
WWE is just like UFC for the athletes in terms of money potential. Champions can make millions as movie stars or from commercial appearances. Some might get flash in the pan money (Nate Diaz, Masvidal). But most make almost no money and basically ruin their bodies and minds always chasing that dream of true stardom. Not forgiving business. I used to cover sports professionally (baseball, football) and I routinely saw the horror stories of failed athletes from smaller sports like boxing, mma, and wrestling with destroyed bodies and souls and little money to show for it.