> how does the Boot partition know, where MorphOS is located,
> to be able to select to proper partition?

"Without arguments MorphOS boots a device that is bootable and has the highest boot priority set. Note that this only applies to partitions that contain a MorphOS compatible bootable filesystem. If a MorphOS installation CD is inserted, it usually has a higher priority than bootable partitions on the HD. The HDConfig tool can be used to set partitions bootable and alter their boot priorities […]."
https://library.morph.zone/Open_Firmware#Boot_Arguments