How to get username after logged in?.I am trying with MSAL (@azure/msal-angular) for Azure Signin

You could use myMSALObj.getAccount().userName to get userName.

function showWelcomeMessage() {
    var divWelcome = document.getElementById('WelcomeMessage');
    divWelcome.innerHTML = 'Welcome ' + myMSALObj.getAccount().userName + " to Microsoft Graph API";
    var loginbutton = document.getElementById('SignIn');
    loginbutton.innerHTML = 'Sign Out';
    loginbutton.setAttribute('onclick', 'signOut();');
}

For more details, see here.