An incredibly rich ecosystem. I would recommend checking projects such as Uniswap, Aave, Sushiswap, Compound finance, Maker DAO, Yearn finance, Balancer, Synthetix, Alchemix, etc. There are over 52 billion dollars worth of capital being used in these projects today https://defipulse.com. Uniswap v3 is an extremely sophisticated dapp being used today https://uniswap.org/blog/uniswap-v3/. Their code is a masterclass in solidity optimizations. dapps on Ethereum are thriving