I tried to generate paper wallet from
https://www.bitaddress.org. I am wondering how to spend the BTC once they are on a paper wallet and how to see the balance of the wallet.
furthermore, it generated several encrypted private keys. does it means, it generated several wallets
For normal endusers, a wallet is basically a piece of software that manages your private keys, and sums up the unspent outputs that can be spent by the addresses generated by the private keys it manages. That's all there is to it.
bitaddress.org generates a private key (preferably encrypts it), calculates the public key using this private key and hashes this one to generate the address. All of this can be done 100% offline. The network doesn't need to know any of this.
You can generate transactions whose outputs can be spent by the address on your paper wallet just like that, the network doesn't need any prior knowledge of your address, nor your public key, nor your private key in order to do this.
Once the address on your paper wallet has unspent outputs in the UTXO set, you can decrypt the private key on your paper wallet ,and import it into (for example) electrum.
Electrum will calculate the addresses, and scan the UTXO to see if there are outputs that can be spent by the address it generated based on the private key. Since you imported the private key into electrum, electrum can now use these unspent outputs to generate and sign new transactions (altough it's recommanded to use electrum's "sweep"-functionality instead...).
When you sweep a paper wallet, exactly the same happens: electrum fetches (and possibly decrypts) the private key, calculates the corresponding addres, scans the UTXO to find outputs, then derives a new address from your existing wallet and generates a transaction to use all unspent outputs that belong to your paper wallet as an input and uses the address it got from electrum as an output.
IF you are uncertain about what happens, i'd recommand you to generate a testnet paper wallet first:
https://www.bitaddress.org/?testnet=trueyou can fund this testnet paper wallet by using testnet faucets:
https://www.google.com/search?q=bitcoin+testnet+faucet&oq=bitcoin+testnet+faucetThen you can use a testnet wallet to sweep the testnet paper wallet:
https://en.bitcoin.it/wiki/Testnet#Wallets (i'd probably recommand the testnet android wallet, or to install bitcoin core and start it with the -testnet argument)