The file
/EFI/APPLE/EXTENSIONS/Firmware.scap
is the EFI extension that shipped with the latest operating system installed on that disk. You can also find it at the following location when booting that macOS system:
/usr/standalone/i386/Firmware.scap
This file contains EFI extensions that your Mac may need to boot, e.g. because it cannot boot from an APFS file system otherwise. Some Macs also require it as without that file they cannot boot from external drives or only from some kind of those. In some cases it must change the state of hardware at boot time otherwise the hardware will not work correctly or not work at all. It is even required in some cases to perform a recovery installation over the Internet or booting over a local network.
If you delete that file, your Mac may fail to boot macOS from that drive. Yet all of the above always only applies to the version of macOS that this file came from. So even if your Mac fails to boot that version of macOS without that file, it may still be able to correctly boot an older version from another partition or disk. And if your Mac is somewhat recent, it will also boot without that file as it already has (most or all of) those extensions embedded into its EEPROM firmware. The older your Mac is, the more likely it will depend on that file being present during boot.
In case you accidentally deleted it and this causes trouble, you can easily restore it as long as you still have a way to mount the EFI partition and copy the file there from another macOS installation. If you deleted it and there is no problem, the system will still restore it on certain occasions (not on every boot but on every system update for instance).
The file
/EFI/APPLE/FIRMWARE/MBP112_0138_B18_LOCKED.scap
contains a copy of the latest EEPROM firmware update installed. When Apple ships an EEPROM update (as part of a system update or stand alone), it is copied to /EFI/APPLE/FIRMWARE/, an entry named efi-apple-recovery is written to NVRAM (run nvram efi-apple-recovery in Terminal to see the content of that entry) and on next boot, the system performs an EFI firmware update (writing the content of that file into the EEPROM).
The EEPROM part of EFI is the part that is loaded directly after boot. It's the part that needs to initialize hardware to the point it can detect keyboard keys pressed, display something on screen, and access the EFI partitions of detected drives. Thus it also contains a file system driver to read that EFI partition as only then it can actually load EFI extensions from there. This is the part of the EFI firmware that is always there and will stay there when your drive is swapped as it is directly "burned" into a chip.
For those who are not familiar with (U)EFI, think of it like this:
The EEPROM EFI part is what used to be the classical BIOS that gets your computer into a bootable state after power cycle. The extension EFI part on the EFI partition is like a boot loader, that takes over when booting the operating system is supposed to start and it may contain any kind of additional hardware/filesystem support relevant to booting systems. Yet a dedicated complex boot loader isn't always needed. Also with the classical BIOS it wasn't always required to have a complex boot loader installed at all on your hard disk; sometimes the BIOS would just read the MBR (master boot record) to RAM and execute the code found in it and the system booted.
On a Windows PC, EFI EEPROM updates would be shipped by the mainboard vendor whereas EFI extensions are shipped by Microsoft through Windows updates (e.g. without an EFI extension, Windows could not boot from a NTFS partition, since EFI itself doesn't know the NTFS file system). Yet in case of a Mac, Apple is both, the vendor of the board and the vendor of the operating system, so Apple ships both kinds of updates.
