Lightning Message Service is used to communicate across the DOM between Visualforce, LWC and AURA. Github URL : Lightning Message Service (LWC + AURA + Visualforce)
I have done account sync between Salesforce (Source) to salesforce (Destination) using Lightning flow. Just visit Blog: https://www.catchontosalesforce.com/p/http-post-via-lightning-flow.html
We can authorize server to access data without interactively logging in each time and unable to share your client secrete. Just visit Blog: https://www.catchontosalesforce.com/p/oauth-20-jwt-bearer-flow-for-server-to.html
I have created a sample javascript app with functionality of Authorization Code and Credentials Flow for Private Clients. Just visit Blogshot: https://www.catchontosalesforce.com/p /authorization-code-and-credentials-flow.html Github : https://github.com/Priyananth-Salesforce/Headless-SF-Authenication--Login-Via-Javascript-App