1- Why SHA2 has SHA224, SHA256, SHA384 and SHA512 variants?
2- Can we say SHA512 more secure than SHA256?
3- Symmetric ciphers use at most 2^256 security level and I saw on the internet people saying how impossible to reach that then Why SHA2 has a SHA 512 variant? Do we need 2^512 security level? İsn't it overkill?
4- İs SHA 512 meant for post quantum world? If it's meant for post quantum world, Why does it not popular? because AES256 already in use by encrypted messengers, password managers.. etc. but I've seen their security whitepapers when it comes to SHA2 most of them use SHA256 instead of SHA512.
5- so if we don't need 512 bit security, Why all the state of the art hash algorithms(e.g. BLAKE2, Skein, Grostl, JH, Keccak) have 512 bit variants?
6- Let's say there's a key derivation function that uses Skein 256 as a underlying hash function, Would it be still secure in a post quantum world? or Would it be better to use Skein 512? to ask it another way, Why Argon2 settled on using BLAKE2b 512 instead of BLAKE2s 256?