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what do you mean by that?

It is interpreted by node software and wallet software, which pass the arguments through the functions (usually reimplemented in their own programming languages), and if -- after running -- there is a single 1 on the stack, they accept the transaction as valid, otherwise they consider it invalid

what was missing from this tutorial, was a way to get the state of the program at any point in time during its execution for debugging purposes

The scriptwiz ide lets you debug your programs by viewing the results at every line of code -- check out https://ide.scriptwiz.app and hit me up on telegram if you have any questions

Maybe one day I'll understand this well enough to write a testnet wallet

Yes! You can do it! I am happy to help you out at every step, just ask :) I can direct you to additional developer resources and things like that