Is there any Demo or tester BTC wallet I can use to transfer between wallets just to see how BTC transaction works? Thank you
Yes many Bitcoin wallets have testnet mode and you can use them perfectly fine with testnet coins.
Some of the wallets I know have testnet option are Electrum, Blockstream, Coinomi, Wasabi, and even hardware wallets like Trezor and ledger have this.
Note that testnet coins have no value and you can get them with faucets if you want to test how everything works.