How can you generate a PFM signal for an LLC converter?
how is PFM usually generated?

You keep mark-space ratio at 50% (optional of course) and vary the frequency of the square wave.

Does this require separate IC's or can I implement this on Arduino /
STM32?

For PWM it's very convenient if you have an MCU with pulse width modulation features. For PFM, in a basic form, you need to set up a loop that that can accept as an input a variable that defines how long the loop takes to execute thus varying the time duration. Counter/timers help a lot and can produce an interrupt when the loop end-count is reached.