How Keycloak validates my user and how to save to my db?

Hi everyone! I have just started exploring Keycloak and have questions about it.

  1. How to handle register event,so i can insert some data to my application database? I couldn’t find proper guides using spring boot.
  2. I created and configured my client,set access type to Confidential.Authentication works fine,but the question is, how Keycloak validates my user? Because i am not explicitly sending access token to my backend server. Does it uses Oauth_Request_State cookie? Or should I validate accesstoken whether it contains client_secret in every request? How Keycloak security flow works?