Foundry and Hardhat

PoC Writing
PoC is the only way to confirm the bug you found is really a bug. Moreover, it confirms the impact of that bug.
A finding report must comes with PoC, otherwise judges won't look at it. For example, here is the Immunefi PoC rules:
My go-to framework for PoC writing is Foundry, whereas some projects still use Hardhat so you should learn that as well.
A list of resources: