I'm a freelancer. How is my tax handling different from a regular employee? I heard about a 3.3% deduction, is that all?

I'm working as an English teacher in Korea, and I'm a bit confused about how my taxes are determined. Can you explain it simply?

Income Tax Withholding Receipt

My E-7 visa is expiring at the end of this month, so I already submitted an extension application at the Immigration Office. However, I need to travel back to my home country for family reasons. Can I leave Korea before the extension is approved?

경력증명서를 어떻게 발급할 수 있을까요?