What happens to FERS when leaving Fed?

I am currently 40 years old + have 15 years of service. If I leave federal service or FIRE before my MRA/30 years of service, what should I do with the FERS retirement money ?

I understand I can withdraw it and move to a IRA. Am I able to withdraw mine AND agency contributions ? Or just my contributions? Is it considered a rollover or a contribution to the IRA ?