Thank you so much for your kind reply.
I think "code fork" is what I want to accomplish.
How does one do it? I can't find much info on it on google
could you provide me any reference please ?
It's when you fork the code of a repository, for example on Github. If you're unfamiliar with basic software development concepts such as forks and version control you should probably start with something else than cryptocurrencies.
What I mean by mainnet is that people can join the mining network and just mine and
contribute to the network.
Anyone can provide their computational power to any network. Question is whether they want to.
If code fork happens, can people just use regular bitcoin wallet (if it forked from bitcoin)
and bitcoin transaction website? Thank you.
If you just fork the code without changing any of the protocol rules, then you simply have a Bitcoin client that interacts with the same blockchain and you can send and receive transactions like everyone else.
If you fork the code and change any of the protocol rules, or start from a new Genesis block, then you're on your own blockchain and won't be able to send transactions to or receive transactions from the Bitcoin blockchain.