Here are some examples I'm aware of, I haven't personally verified these.

Uniswap: https://uniswap.org/audit.html

Sushi Swap: https://github.com/peckshield/publications/blob/master/audit...

Alpaca Finance (high complexity in my opinion): https://docs.alpacafinance.org/transparency

A lot of projects provide their source and you can verify that what you're interacting with matches the source.

Not sure what you mean by source, but its fairly easy to find out if any official audits were done as this is always announced and documented by the involved parties (for example, I couldn't find any detailed audit info for Poly). It's not like the audit is mandated by law, its initiated by the SC owners to put the investors at ease. A lot of companies like Certik, Consensys, ZK Labs, etc. provide auditing services for this purpose explicitly.

High quality protocols like PoolTogether for example have this: https://docs.pooltogether.com/security/audits-and-testing