This post is somewhat old, but I have used it as a guide when implementing IdP initiated login. The example is specific to Okta SAML, but it shouldn’t be too hard to adapt it to other IdP types: https://www.lisenet.com/2020/keycloak-with-okta-idp-initiated-sso-login/
The Keycloak docs are here, but I’ve found them fairly hard to follow: keycloak-documentation/idp-initiated-login.adoc at main · keycloak/keycloak-documentation · GitHub