The standard PIN code can be found on the plastic card, which was holding your SIM card or in the email with the QR code you received for the installation of the eSIM. You preferably change it by a code you only know.
How to change your PIN code? If you want for example the code “1375” as a new PIN code, you enter **04*oldPIN*1375*1375# on your smartphone and press the dial button. The PIN code is now changed to your new code.
For security reasons, you better do not choose sequential numbers (1234) or same numbers (2222).