VPNs support multiple authentication methods:
Pre-shared keys: Both sides know a secret password. Simple but difficult to rotate. Fine for site-to-site tunnels between your own devices.
Certificates: Each peer has a public/private key pair signed by a trusted CA. More secure and scalable than PSKs.
Username/password: For remote access VPNs. Often combined with MFA.
RADIUS/LDAP: Centralized authentication against Active Directory or similar.