Strike users or Strike representative. Can you explain what happens under the hood?

Not sure if it is the right place to ask, but I can't find a sub-reddit of Strike.me to ask for clarification.

I read their Medium articles and looked on the internet and kinda understand the way they settle the international transfer. In short for a scenario when Alice sends Bob $100 from US to Germany, for instance:

• Alice is debited $100 from her bank account
• $100 worth of Bitcoin is bought, then sent over EU over lightning network.
• That $100 worth of Bitcoin is sold to EUR, then credit to Bob.

They mention it is zero fee, and it seems to have little slippage. So if you see on Google $100 worth €92, Bob would receive €92.

I understand why they choose lightning network to settle down the payment. But my questions here:

• whose Bitcoin Strike is buying from? From Strike users who is selling?
• Then, who is buying Bitcoin in Euro? It should be bought in EUR so there is a little slippage, right?

If it is the case that Strike users buying and selling, it appears like Strike is operating the order book. Lightning network and Bitcoin as a very liquid asset help them to make this possible.

Can anyone confirm?