You can start the analysis of DeFi applications with the research of the blockchain. It can be, for example, Ethereum because most existing DeFis are based on it.
You could start your research with the Etherscan
platform or you could take a look at CoinGecko.
Also, there are project chats where you can find the answers.
You can find more detailed analysis of smart contracts at Dune
Platform developers decrypted Ethereum contracts and you can find out more via SQL requests.