Transaction Fee on Side Chain

I watched a whiteboard video about side chain.
It was a great video! So interesting!
But I can’t understand about interoperability.
When two different currency chain connected, how transaction fee collection work?


Cardano SL -> Cardano CL
It’s very easy case.
Both SL and CL collect tx fee by ADA.

Bitcoin -> Cardano CL
This case is difficult.
Bitcoin chain collect BTC burning tx fee by BTC.
How about BTC minting tx fee on Cardano CL?
Is it BTC or ADA?

What will happen if I replace BTC with my own coin in example above?
I issue great amount of my coin and connect chain to CL.
Then I can create infinite minting tx of my coin on CL.
It’s DDoS atack! Must not allow it.

So, “all tx fee on the chain is its native token.” is correct answer?