A quick note on capitalization: Bitcoin with an uppercase B refers to the blockchain network and protocol, while bitcoin with a lowercase b refers to the cryptocurrency that is used on the Bitcoin network. BTC also refers to the bitcoin token.
Ethereum was created to act as a decentralized world computer that uses smart contracts and allows developers to build on top of the core blockchain.
Ethereum has been the leading blockchain when it comes to DeFi, with many of the biggest dApps, DAOs and DEXs built on it, creating smaller cryptocurrency economies within the Ethereum ecosystem.