it depends on what part of it you want to learn, it is too broad.
there is bitcoin wiki (https://en.bitcoin.it/wiki/) that has almost everything and is nearly up to date too.  there is developer's reference (https://bitcoin.org/en/developer-reference) which is similar but is easier to use. some of the missing parts can be found among BIPs (https://github.com/bitcoin/bips). for some things that aren't documented or not properly documented you'll have to look at the source code.
for the cryptography part you'll have to find other documentations. for example here is a simple explanation of elliptic curve cryptography https://blog.cloudflare.com/a-relatively-easy-to-understand-primer-on-elliptic-curve-cryptography/