As I understand each transaction is encrypted, how is that decryption done by the miner for validation without a private key?

That is incorrect. Transactions are not encrypted. No part of the Bitcoin protocol or consensus rules specify that anything should be encrypted. In fact, nothing can be encrypted as the protocol relies on the fact that everyone is able to read all blocks and transactions in order to validate them.

You may be confusing digital signatures with encryption. Digital signatures in Bitcoin are used to ensure that a person is allowed to spend some coins. Signatures are used as a proof that the person who created a transaction is allowed to actually make that transaction and spend the coins that it spends.