using System.Collections.Generic;
public static void Main()
new () { id = 1, value = 88 },
new () { id = 2, value = 222 }
new () { id = 2, value = 77 },
new () { id = 3, value = 666 }
var c = aa.Join(bb, a => a.id, b => b.id, (a, b) => new { Id = a.id, Value = b.value });
Console.WriteLine(JsonSerializer.Serialize(c));