I think it would be worth understanding how Bitcoin works first. It's designed to be more robust. It doesn't change as often (some criticize it due to that), however, it could also be seen as a strength. You could read the Bitcoin white paper [1]. If you wanted to learn more about programming Bitcoin, you could also check out Mastering Bitcoin 2nd Edition - Programming the Open Blockchain by Andreas M. Antonopoulos [2].

Ethereum was created after Bitcoin. It was designed to be more of an open programming platform. It was designed to be Turing Complete and enabled the use of "smart contracts". The DeFi space runs on smart contracts and Ethereum kicked off DeFi. This is the Ethereum white paper [3]. Mastering Ethereum by Andreas M. Antonopoulos & Gavin Wood is another book you could check out [4].

Here are a couple of introductions to DeFi, as well [5][6].

The hack that everyone is discussing here was on Poly Network, which a layer 2 solution for Ethereum [7]. Layer 2 solutions were created due to lower the high gas fees on Ethereum, as well as increase transaction throughput. Ethereum itself is also working on moving to Ethereum 2.0 which would help address the issues that layer 2 solutions are trying to solve [7].

Edit: Poly Network actually appears to be more of a bridge between different networks (Ethereum, Polygon and Binance Smart Chain) [9], so it's worth noting that this wasn't a direct hack on Polygon, which is a layer 2 solution for Ethereum.

[1] https://bitcoin.org/bitcoin.pdf

[2] https://github.com/bitcoinbook/bitcoinbook

[3] https://ethereum.org/en/whitepaper/

[4] https://github.com/ethereumbook/ethereumbook

[5] https://ethereum.org/en/defi/

[6] https://blog.coinbase.com/a-beginners-guide-to-decentralized...

[7] https://www.gemini.com/cryptopedia/polygon-crypto-matic-netw...

[8] https://ethereum.org/en/eth2/

[9] https://www.reddit.com/r/CryptoCurrency/comments/p1qfdo/psa_...