1. From a consensus perspective, how is Ethereum different from bitcoin?
Ethereum’s proof of work is different as transfers do not look at UTXOs, but rather update the accounts’ balances directly. There is no traceability of ETH other than the sending/receiving addresses. Also, block sizes and average block times differ. Lastly, GAS is used to create proportions for different actions in a smart contract (adding, dividing, etc.), which differs from Bitcoin which does not care about anything other than transaction fees. ETH intends on moving towards a PoS model in the future.
2. How are smart contracts executed in Ethereum compared to in a normal computer?
Smart contracts are executed using the EVM (ethereum virtual machine), which ensures that the smart contract isn’t trying to perform any malicious activity on the node’s computer. The node keeps track of the smart contract and the current state of said contract.
3. What are the benefits of executing the code on many nodes instead of just single computer/server?
No central point of attack, making the network more secure. It also provides the gateway to immutability as any changes to records already agreed upon by the nodes (balances & smart contract states) would be identified by the other nodes supporting the network.