That’s for mounting, yeah, but when it comes to interacting with the hardware, Linux itself uses letters for some types of devices. For example, serial-connected ones (e.g. SATA internal drives, USB external drives) are /dev/sdx (x being a letter from A-Z). I don’t know what happens when all letters are used up though, maybe someone can chime in there? NVMe uses numbers it seems - my boot drive is /dev/nvme0n1
There are other ways to access devices and partitions besides that though. I just had to put EndeavourOS on a flash drive and the Arch Wiki recommended doing this by targeting the drive via /dev/disk/by-id/, which lists connected drives by name, connectivity and serial number.
Pretty much, yeah. I think Windows uses something like \\PhysicalDisk0 internally, then shows it to the user with lettering.