You will need to provide your own SD card. It needs to be formatted as FAT32.
Initially, the printer does need some wi-fi access to the Bambu server in order to install X1Plus. The printer must download a special version of the internal firmware(known as Firmware R, which is based on V1.06.00) that has been modified to transfer control to the enhanced firmware on the SD card.
To obtain access to that firmware, you need to log in to their website via browser and agree to the Third Party Firmware Guidelines. Then Bambu makes that version available for “downgrade” through your Handy app.
Then later, as part of the X1Plus installation, a more current “base” version of the Bambu firmware is downloaded from their servers to the SDcard.
AFAIK, V1.06.xx is not available for offline installation via SDcard . I’m currently using v1.08.02 (the first to allow offline upgrades) as a base firmware with my X1Plus v3.0. It’s not yet clear to me if X1+ will allow for fully offline “base” upgrades in the future.