You can find sample implementation of RP Iframe on WSO2 OIDC sample — Playground.
Please find the source code in [1].

OP Iframe sample implementation can be found in [2]

[1] https://github.com/wso2/product-is/blob/5556b18654e0ad24df6bf6a767a8987bc16ff8d6/modules/samples/oauth2/playground2/src/main/webapp/rpIFrame.jsp

[2] https://github.com/wso2-extensions/identity-inbound-auth-oauth/blob/master/components/org.wso2.carbon.identity.oidc.session/src/main/resources/op_iframe.html