bitcoin core – Why are there multiple payment addresses associated with a private key?

Concerning the output of dumpwallet, this is the best answer:

dumpwallet output documentation & explanation?

About your questions:

Why does a private key address correspond to two payment addresses?

Addresses are instructions from your wallet to the paying wallet.

These instructions are divided in 2 parts: a prefix and some string.

If the prefix is:

• “1” this means your wallet wants the receiving coins to be locked in a P2PKH script (Pay To Public Key Hash), and the hash of your public key is the string part of the address;
• “3” this means your wallet wants the receiving coins to be locked in a P2SH script (Pay To Script Hash), and the hash of the script is the string part of the address;
• “bc1q” this means your wallet wants the receiving coins to be locked in a Segwit script, and the hash of the script is the string part of the address;
• “bc1p” this means your wallet wants the receiving coins to be locked in a Taproot script, and the hash of the script is the string part of the address.

So, the same exact private key can originate 4 different addresses.

In the 1.txt file, the receiving address beginning with 3 is followed by 00…… This code should not be a private key. What is it?

It’s the P2SH address for that private key.