How do custodial lightning wallets work?
I am sorry to say but your understanding is wrong.

When you send x btc to Bluewallet you don't open a channel. You just give them your Bitcoin. Bluewallet and other custodial wallets run one (or possibly several) lightning nodes and allow you to use it with up to the amount of btc you have sent to them before.

So as long as Bluewallet users send each other payments no channel is needed as everything can just happen virtually in their accounting software. Also in a custodial wallet you can receive btc via lightning without providing liquidity first (as with your non custodial fullnode)