by the miner nodes which make sure that a transaction is valid.

by every single node in the blockchain network executing the same transaction.

by a cryptographic secure signature algorithm called ECDSA which makes sure that cheating is impossible.

it's good to use the block timestamp, as this is always different.

it's good to use the block hash as this is clearly always very different.

it's good to use the RANDAO smart contract.

it's not possible to have a random number in a deterministic environment such as the Ethereum blockchain.

Address.call() is used for calling other contracts using the scope of the called contract in terms of storage variables. Address.delegatecall() is used for libraries, which uses the storage variables of the contract who called. Libraries are a great way to re-use already existing code and delegatecall can make sure that no storage is used from the library, instead it looks like the code is directly copied into the calling contract.

Address.delegatecall() is used for calling other contracts using the scope of the called contract in terms of storage variables. Address.call() is used for libraries, which uses the storage variables of the contract who called. Libraries are a great way to re-use already existing code and call() can make sure that no storage is used from the library, instead it looks like the code is directly copied into the calling contract.

a 20 bytes value

a 32 bytes value

a string

a 20 characters long hex number

to bytecode that can't be understood by humans.

to bytecodes which are essentially opcodes running instruction by instruction.

string1 == string2

the internal function "str_compare(str1,str2)"

you can't directly compare two strings, but one method would be to hash both strings and compare the hashes.

bytes32(string1) == bytes32(string2)

will cascade exceptions and address.transfer() will return a false on error.

will return false on error while address.transfer() will cascade transactions.