Apple II ProDos was loaded from a “system disk.” The actual boot loader was burned into ROM but all it did was direct the primary disk drive head to Track 0 Sector 0 and start a load there, then JMP to the load-point address.
If I remember my Apple II internals ...
Yes, that was the procedure - all disk handling had to start from the system disk. The rest of the operating system, like keyboard, display and Basic was on ROM.