I've read several hadiths on apostasy in Islam and how if one leaves Islam willingly then he has to be killed and theres 0 chance that his repentance will ever be accepted by Allah (SWT). If someone leaves, they can never come back? I thought Allah (SWT) forgives every sin

No, if a person repents truly Allah forgives them. Yes, if the apostate dies a non-believer and never repented, then they are not entitled to God’s forgiveness. But as long as he repents and returns to faith, Allah forgives.