Does FPGA have internal clock source?
Most FPGAs have a PLL clock synthesis block that generates the clock/s you need from some kind of source. That source may be an external crystal plus amp circuitry in the chip, or an external resonator, or an on-chip resonator, or something else, or a combination/choice of multiple options.

The only way to know the real answer for a particular chip, is to read the data sheet of that particular chip. When you work with any piece of electrical component, you should get ahold of, and read, the data sheet for that component. This, btw, is also true for the Atmega328p on the Arduino boards -- if you haven't yet read the Atmega datasheet, then you're probably not yet ready to move on from AVR MCUs to FPGAs ;-)