How could SD card be read if there isn't drive letter assigned to it and it doesn't show in Disk Management?
I've been told that an SD card appears as a file on Linux, macOS and Windows. But, I have no clue what this is or how to access it.
I can speak for Linux and the same probably applies to the Mac.
Whether an SD-Card appears as a named device, to which fdisk may optionally be applied, and with the result mountable as a filesystem hence able to support files, depends on what comes between the card and the host computer.
If you have a USB-connected reader, there will be a chip or a "black blob" inside it. In either case that will be a microcontroller which presents itself as a USB storage device at one side, and on the other side talks either SPI or the MMC protocol (see Wp page).
If the OS doesn't recognise the card as a storage device, then you need to break in at the engineering level using either SPI or MMC. A PC is not well-suited for that since you need something like a CH341A chip to generate SPI to a (dumb) card adapter, a Raspberry Pi is somewhat better since it supports SPI directly.
However there are /far/ more people doing this sort of thing using hardware comparable with an Arduino, I have in the past used Arduino code in an attempt to see if there was any life at all left in a card that was physically damaged.
I'd also note that over the last couple of weeks I've been using an SD-Card attached to an RP2040: once I sorted out the non-standard pinning used by the prototyping board I'd got talking to the card using SPI was no real problem.
I have the same query as you, about accessing an SD card that the OS doesn't recognise the format but is not broken.
This is the second or third time I've said this in this thread: look at it using something like an Arduino since you're likely to need to get at it at a lower level than off-the-shelf PC interfaces support. Or at the very least use an RPi rather than a PC, but at that point expect to have to port Arduino code onto it since that's where all the work is being done.
MarkMLl