Each crypto operation (“verify the signature of this message”, “encrypt this chunk of data with this key”) is covered by any one of a number of libraries and if you’re writing your own implementation you’re probably doing the wrong thing.

For the system that you’re trying to build (messaging system, secure boot, HDD encryption, etc) working out which one you need to when is where the difficulty is. What is the overall design of your system - how do keys get exchanged, what is important to protect etc.