Use of frontdoor.jsp to Log in to Salesforce

Use frontdoor.jsp to give users access to Salesforce from a custom web interface, such as a remote access Force.com site or other API integration, using their existing session ID and the server URL. For Experience Cloud sites, use a web-scoped access token. Here's an example of what the URL would look like:

https://instance.salesforce.com/secur/frontdoor.jsp?sid=session_ID&retURL=optional_relative_url_to_open

Retrieve the session ID 

1. The access_token from an OAuth authentication. Note that one of the scopes specified when you create a connected app must be web or full.
2. The LoginResult returned from a SOAP API login() call.
3. The Apex UserInfo.getSessionId().