Do you know if it is Windows or Android based? If it's Windows based, it's probably something like W11 IoT Enterprise GA or even a fixed year like 202X. If that is the case, it is likely based on iDP or HDMI internally. If it is based on HDMI with a embedded PC inside, you can probably run it straight to a PC with an HDMI extender. Failing that, get the part number from the LCD and look for a driver board like people use to convert broken laptops and AIOs to monitors. Some are also known to use MIPS, no matter the OS base.
If it's Android based, then it is based on a protocol like LVDS or iDP and you will need a driver board that takes the signal and makes it "PC friendly". Again, LCD will tell you what you need. I would not expect it to be possible to convert the "customer display" - let that one go. It's thoroughly nonstandard as it's meant to be part of the terminal.
EITHER WAY YOU WILL KILL IT'S USABILITY AS A TERMINAL. Once you mess with the hardware it will kill the encryption keys used to handle transactions! I would also not use it in a business, like... at all, even in the back. Take it home and use it as a ancillary display.
This is what you will need to convert to -- DP or HDMI, I would avoid VGA: