PS1 memory card converter
A PlayStation memory card is 128 KB, and most emulators keep it as is under different names: .mcd in DuckStation, .srm in RetroArch, .mcr in ePSXe. DexDrive's .gme and the PSP's and Vita's .vmp add a header. Turn any of them into the card your emulator wants.
It finds the raw card inside the file (removing the 3,904-byte DexDrive header or the 128-byte PSP and Vita header), checks that it starts like a PlayStation card, and saves it with the extension you picked. A raw card is only renamed.
For one game's save rather than the whole card, open the card in MemcardRex or DuckStation's Memory Card Editor.
DuckStation gives each game its own card by default, named after the game's title, such as Final Fantasy VII (USA)_1.mcd. Name the converted card the same way, and put it in its memcards folder.
Converting is a one-off. If you play the same games on more than one device, SaveSync keeps the saves in step between them through RetroArch's built-in Cloud Sync, and keeps the 20 newest versions of each.