Wie berechnet man die Ausgabe von Bitcoin?

Could you elaborate on how the issuance of Bitcoin is calculated? I'm particularly interested in understanding the underlying mechanism that determines the supply of this cryptocurrency. Is it a fixed amount that's predetermined or does it vary based on certain conditions? Additionally, how does the mining process contribute to the issuance of Bitcoin? I'd appreciate a concise yet thorough explanation of the process involved in calculating the issuance of Bitcoin.