The process for unlocking the bootloader on an Android phone varies depending on the manufacturer and model. Here's a general outline:
Yes, in most major markets. In the US, the Unlocking Consumer Choice and Wireless Competition Act (2014) made it legal to unlock your phone once your contract or installment plan is paid off. The EU also supports carrier unlocking. Always check your local laws, but for most readers: legal, as long as you own the device or have paid it off. unlock a cell phone