ParanoidNemo so I can just flash GrapheneOS and on reboot the eSim will be already setup and working. Where is the truth?
The "truth" is in the statement above. If you already have an esim installed, it will stay even after flashing GrapheneOS.
See here: https://grapheneos.org/usage#esim-support
By default, GrapheneOS has always shipped with baseline support for eSIM, where users can use any eSIMs installed previously on the device.