A quick question. I used web3js lib to create an app in which a user can create his own account and also a wallet. My question is is that wallet can be used only with the ethereum coin transfer or can I use to store or transfer btc as well?
web3.js is strictly Ethereum specific library. It shares some of the cryptographic library dependencies with Bitcoin, but if you plan to create a non-custodial Bitcoin wallet there are better JavaScript libraries as starting points out there.