Bitcoin transaction generally require at least one signature in order to be valid.
Signatures prove that the intended recipient(s) of the funds being spent were involved in the creation of the transaction.
Each signature is generated using a private key and indicates that the owner of that private key approves of certain data in the transaction.
This page describes how these signature are generated, given data to be signed and a private key.
For details on what data is signed when signing a transaction and how the signature is formatted, see Transaction Signing.
Nexa uses Schnorr for transaction signatures.
Schnorr signatures have been accepted in Bitcoin Cash since HF-20190515.
Details on the implementation of Schnorr used in Bitcoin Cash can be found here.
Of particular importance are the following points: