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 ...
Is this even possible?
You can move wallet contents from almost any Bitcoin wallet software to any other Bitcoin wallet software that supports the same standards. For example, if the first wallet supports Bech32, you will need the second wallet to also support Bech32.
There are only a couple of small pieces of data that need to be copied ...