I been learning about ethereum and smart contracts. I have seen many ICO's are developing crowdsale smart contract based on Ethereum platform. But i figured out they are sill accepting BTC, Litecoin etc. Bitcoin and other coins will not be accepted by Ethereum crowdsale smart contract except Ether. So I been wondering

• How multiple coins are accepted in ICO's whose smart contract is based on Ethereum ?
• How exact number of tokens are sent to one who pay crypto coins and How this is done ?

I don't know if this questions is kinda silly or too technical. But i want to know its working.

Please point me to any documentation sources to learn about everything.