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If I wanted to do testnet4 miniscript and taproot fun, how do? 

 Check out BDK playground https://bitcoindevkit.org/bdk-cli/playground/

You can also use BDK cli and manually point it to a testnet 4 node 

 What is the "hello world" of taproot miniscript? 

 Key A (key path), or Key B, and 100 blocks (script path)