One question on this topic interests me: In BitTorrent, do nodes that bootstrap using different bootstrap servers, ever see themselves in one single connected graph?

Since there are many different bootstrap servers for mainline DHT in BT, how is it ensured that nodes from two or more different bootstrap servers know about each other?

Is it possible (and how often does it happen) that a node that boostraps using server A will never find nodes that bootstrap using server B?