What's an EFI?
An EFI partition is where a computer stores the bits needed for the system to boot. In the case of a Hackintosh, it's where OpenCore and all of the drivers and configurations that OpenCore needs to start your system are stored. As you are building your Hackintosh, you are going to become intimately familiar with this partition and its content. Older computers that don't support EFI boot OpenCore using a legacy stub. Here's how you mount it.
First, inspect your partitions and find your EFI. If you have two hard disks installed, be careful to determine which one contains the correct EFI as the IDs themselves can change on every startup.
diskutil list /dev/disk0 (internal, physical): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *500.1 GB disk0 1: EFI EFI 209.7 MB disk0s1 2: Apple_APFS Container disk1 499.9 GB disk0s2 /dev/disk1 (synthesized): #: TYPE NAME SIZE IDENTIFIER 0: APFS Container Scheme - +499.9 GB disk1 Physical Store disk0s2 1: APFS Volume macOS 199.9 GB disk1s1 2: APFS Volume Preboot 46.5 MB disk1s2 3: APFS Volume Recovery 510.3 MB disk1s3 4: APFS Volume VM 20.5 KB disk1s4
In the example above, you'll note that you have one internal disk and one synthesized disk. The synthesized disk is a container that holds all of your operating system bits. The source for this container can be found at /dev/disk0s2. You should also note that the EFI partition is located on the internal physical disk at /dev/disk0s1. This is the partition that you should mount for OpenCore. Here's how you would do it.
sudo mkdir /Volumes/EFI sudo diskutil mount -mountPoint /Volumes/EFI /dev/disk0s1
That's it, you're ready to work on your OpenCore installation! Simple! When you've finished editing your OpenCore folder, you can eject it in Finder or from Terminal like this.
diskutil unmount /Volumes/EFI
What are EFI drivers?
EFI drivers are used by OpenCore to bootstrap macOS and start the process of loading the kernel and services that get you to your desktop. These are found under OC/Drivers.
So what's LEGACY?
Another great question! For many years computers used a basic data structure to identify where your data was stored, and how to boot your operating system. This method added a small amount of data to the very start of your hard drive. Beginning in 2004 and adopted later by vendors, Intel established a new method of booting a computer called UEFI that was 32bit and provided significantly greater flexibility. If you have support for UEFI, you should use it. If you don't however, OpenCore will still accomodate you. You will also access your OpenCore directory the same way, the major difference is how the system recognizes and bootstraps the OpenCore bootloader.
As Legacy is well..Legacy..these types of installations usually require a lot of additional work which is not covered by this guide.