[rawtxapp]

The 2nd layer always maps to the settlement layer as in you're still transacting in Bitcoin, you just don't broadcast the signed transactions to the whole network because you expect to continue your transactions.

You can use eth or any of the thousands of other crypto-currencies, Bitcoin benefits from being the most secure and also having the largest network effects due to being the earliest. When it comes to money, security is the most important thing and Bitcoin offers the most secure settlement layer.