How was bitcoin private/public key created in 2009, I observed in the blockchain some bitcoin transaction took place early 2009 Feb - April. In the original bitcoin core in github the first bitcoin core was released September 2009. My confusion how were the users able to generate the address ?
The first version of Bitcoin (0.1.0) was released on January 9th 2009. You can find all versions of the software at the Satoshi Nakamoto Institute. Bitcoin Core as a project didn't emerge later, after more contributors started writing code. The name "Bitcoin Core" wasn't used until version 0.9 of the project, which was released in 2014. The project didn't use GitHub in the beginning either, but Sourceforge.
To summarize: the code that Satoshi released in early 2009 had functionality for generating private keys and signing transactions. Bitcoin Core is a later project, and GitHub does not have the complete history.
Prior to BIP32, most wallets simply generated keys from random data provided by the operating system (or some other entropy source).
A handful of wallets implemented a brainwallet system, in which keys were derived from some kind of password, but they quickly fell out of use after it was discovered that most people selected easily guessable passwords which resulted in excessive loss of funds to theft.
The procedure to generate bitcoin addresses and their corresponding private keys is fairly simple, as can be seen at https://en.bitcoin.it/wiki/Technical_background_of_version_1_Bitcoin_addresses. Early bitcoin addresses and keys could easily have been generated using a series of command-line commands or simple scripts that performed these steps.