Get users last accessed time

For recording the user active time, you could try Middleware which will be called for every request.
Here are steps.
1.Add field to ApplicationUser which is store the last accessed time

public class ApplicationUser : IdentityUser
{       
    public DateTime LastAccessed { get; set; }
}

2.run command add-migration LastAccessed and update-database
3.Add middleware to update the last accessed time based on the user name

        app.UseAuthentication();
        app.Use(async (context, next) => {
            await next.Invoke();
            //handle response
            //you may also need to check the request path to check whether it requests image
            if (context.User.Identity.IsAuthenticated)
            {
                var userName = context.User.Identity.Name;
                //retrieve uer by userName
                using (var dbContext = context.RequestServices.GetRequiredService<ApplicationDbContext>())
                {
                    var user = dbContext.ApplicationUser.Where(u => u.UserName == userName).FirstOrDefault();
                    user.LastAccessed = DateTime.Now;
                    dbContext.Update(user);
                    dbContext.SaveChanges();
                }
            }
        });