It was distributed on paper tape. You needed a teletype with a paper tape reader to run it. Basically you would manually enter a bootloader using the switches on the Altair's front panel, and the bootloader would read BASIC off the tape and into RAM. If the checksum passed, it would then jump into BASIC. Here's a video of the process if you're interested: https://youtu.be/TxU_3dEJ2nM?t=1013