What is the difference between Bitcoin & ETH?

As a cryptocurrency enthusiast, I'm curious to understand the fundamental differences between Bitcoin and Ethereum, or ETH. Could you elaborate on the key distinctions? I've heard that Bitcoin is focused primarily on being a decentralized digital currency, while Ethereum aims to be a platform for decentralized applications. But I'm interested in delving deeper into the technical differences, like their consensus mechanisms, scalability, and the role of smart contracts. Could you explain these in a way that's accessible yet comprehensive? Additionally, how do these differences impact their use cases and the overall crypto ecosystem? I'm eager to gain a clearer understanding of the two leading cryptocurrencies.