Let's take this token and one of the transactions of it:
https://solscan.io/token/AbnTggpTujbdAiJtyhH9WtK2CqXk44W7GipyJXkopBDd
Looking at one of the transactions, will one always be able to create and, possibly, execute, an identical transaction? Is there always enough info in there, or at least is there always a possibility to deduce needed info from Solana?
Or can there be info that can't be found anywhere publicly?
Particularly, what's createAssociatedAccount? Where to look for it at all?
If you have all of the required private information, you can absolutely create an identical transaction.
Let's look at https://solscan.io/tx/2EkndYmogcyHEcckfvaYmLXcqvvWC1AutSZE77y1gsDsCSS15PQ5sWV4NNZwqx7qUvNkN7H8GhmDZ8hE35dj98fP as an example. This transaction contains an instruction to transfer 620 tokens of mint AbnTggpTujbdAiJtyhH9WtK2CqXk44W7GipyJXkopBDd from AqUzLDLJ1LhLv9gERjEn78j4PMEpvyWJpkSTuWQ54KwD to EdkpJiv7hgspKcEeYL9JmDaipRQH6rd9xTySZwxB3Whg, signed by GrX7rZAjCFH9htKfuxYEMGNSg1mDnr5dmwYhBgVmgdvR. The only private part here is the signature by GrX7rZAjCFH9htKfuxYEMGNSg1mDnr5dmwYhBgVmgdvR, so you would need the private key in order to sign a new transaction.
On the other hand, if you duplicate the exact transaction as is, and resend it to the network, it will get dropped as a duplicate. You need a new signature referencing a new blockhash (https://docs.solana.com/terminology#blockhash), which requires the private key.
As for createAssociatedTokenAccount, it's a standard for associating a token account to an owner. More info can be found at https://spl.solana.com/associated-token-account