MacBook Pro OS drive from a MacPro

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MacOS X after 10.6 Snow Leopard installs everything you could ever need for any appropriate Mac whenever you install macOS. "The right stuff"for your specific machine is loaded at runtime.

Issues you might face are if you try to run MacOS earlier or later than the other machine allows, or you radically change device types (such as Installing NVME type SSD drives only supported in 10.13 High Sierra and subsequent.

The Mac Pro 5,1 was released in July 2010.

Its operating system range is Mac OS X 10.6.4 (system specific, and does not support retail 10.6) through 10.14.6 if you have the right graphics card.

The mid-2012 MacBook Pro supports 10.7.5 through 10.15.7.

Any system that they share in common can be used between them from an external hard drive.

You can't install 10.6.4 though on a mid-2012 MacBook Pro.

HFS Extended Journalled runs on all drives up to Mac OS 10.12.6.

APFS is the only format that supports SSD drives starting with 10.13.0.

10.12.6 and earlier booting drives can't see APFS formatted drives if mounted via USB, Thunderbolt, or Firewire, but if they are file shares you can move data over a network.

You don't need to open anything. Target Disk Mode on Firewire or Thunderbolt adapted Firewire can be used between the two machines can be used if the formatting is the same. Hold the T key at startup.

The readily available article on the subject of Target Disk Mode only goes back as far as High Sierra, but this feature worked LONG before that. It was first available for Macs with SCSI drives.

Transfer files between two Mac computers using target disk mode - Apple Support

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