Say if someone owns 10 Bitcoins which is not declared and government doesn't know about it (earned/mined etc but did not buy) and now that someone want to avoid paying tax on cashing it out. Can he simply move to countries where there is 0 tax like (UAE) specifically dubai and setup a company there and cash out crypto over years in dubai bank account and later decide to move to his home country with all that money. Since dubai has 0 personal tax and his home country can't charge tax on it because of non-resident of home country. Is this possible?
Very much possible! But the amount of bitcoin needs to support your expenses in tax-free countries like Dubai. Also, each country has its own definition of tax systems so first find out how many days you have to spend abroad so that your own tax department considers you as a non-resident and doesn't impose taxes based on that tenure. In my country, a person needs to stay abroad for 180+ days to be considered a non-resident. So, find out that information for your country and plan accordingly.
Also, many tax-free countries will require you to furnish your income statement from businesses you are running there. Not for tax purposes but to stay compliant with the international anti-money laundering law. So make sure you have a real-world business to back up your numbers. If you get these things right, you are good!