Most countries have their own law about paying taxes on casino winnings. If you are not sure of percentage or limits on tax deduction on winnings, please ask your local tax authorities.
Online casinos follow strict rules regarding privacy and age restriction on gambling. To be sure that you are the one you are saying and that you are of authority, they need to collect the necessary data according to their license. If such schemes were not in place, it would be very easy for scammers to create an account in your name, or for people under the age of age to pass through the check.
Yes, there are usually limits to how much you can withdraw, Especially if there are bonuses with no deposit you have won. Otherwise, they can operate with both daily, weekly, and monthly withdrawal limits.
The biggest winnings are usually found on progressive jackpot slot machines. You can see jackpots at tens of millions of dollars on Microgaming's Mega Moolah, or NetEnt's Mega Fortune several times a year.
Most of them support Euro, Pound, and Dollars as currencies on the games. They also offer other currencies as well like Scandinavian currencies. Some even support crypto like Bitcoin.