This all is so deep, and I'm so sorry I cannot contribute to the questions you've got, but I feel like never coming back on account of "I know fuck all about this". Is this what people call 'Imposter syndrome'?
There's a lot to know, but you can build up to it!
One thing you'll learn about are oracle attacks, in which a bad protocol or implementation can leak secret data to the attacker bit-by-bit (sometimes literally bit-by-bit) in various ways. There are lots of those and they're sadly easy to create and a lot of progress in cryptographic engineering has been about finding ways to make sure oracle attacks can't occur in various real-world deployments.
There's a lot to know, but you can build up to it!
One thing you'll learn about are oracle attacks, in which a bad protocol or implementation can leak secret data to the attacker bit-by-bit (sometimes literally bit-by-bit) in various ways. There are lots of those and they're sadly easy to create and a lot of progress in cryptographic engineering has been about finding ways to make sure oracle attacks can't occur in various real-world deployments.