PS4/5
Dead Cells on the PS4/5 is shipped as a signed PKG. Inside of the PKG, there are native interfaces for all trophies (achievements), alongside a SELF (Signed ELF) binary that acts as the main entry point (eboot.bin) and the obligatory res.pak, which seems to be V0 but could possibly be V1.
Working with save data
Emulator users
If you're running Dead Cells in shadPS4, your save file is stored under the emulator's data directory at home/<user-id>/savedata/<title-id>/SAVEDATA00/ (on Linux this is typically ~/.local/share/shadPS4/home/1000/savedata/CUSA10484/SAVEDATA00/; on Windows, look under %appdata%/shadPS4/). Inside, user_0.dat is the actual save data, alongside options.json and the sce_sys/sce_backup directories that real PS4 hardware also produces.
CUSA10484 is the title ID observed for one regional release of Dead Cells - other regions may use a different CUSA***** ID. If your save isn't under that folder, check for other CUSA* directories under savedata/.
Real hardware
On real PS4/PS5 hardware, save data lives in the same layout as above: /user/home/<user-id>/savedata/<title-id>/<savedata-dir>/, with the encrypted save container under that folder's sce_sys/ directory.
Official method (no jailbreak required): In the system Settings, go to Application Saved Data Management → Saved Data in System Storage → select Dead Cells → Copy to USB Storage Device. This writes the save to PS4/SAVEDATA/<user-id>/<title-id>/ on the USB drive, but the files are encrypted and signed to your account/console - they can't be read or edited directly without further decryption.
Jailbroken console (CFW like GoldHEN): Enable the FTP server (Settings → GoldHEN → Enable FTP server) and connect to browse /user/home/<user-id>/savedata/<title-id>/ directly - this mirrors the same folder layout as the shadPS4 save above. Homebrew tools like Apollo Save Tool can also export already-decrypted save files directly on-console.
PS5 jailbreaks are much more limited and firmware-dependent; if you've confirmed an equivalent process there, contributions to this doc are welcome.
eboot.bin
This file, as previously mentioned, is a signed ELF that contains the HL/C compilation of the game. Sadly, this ELF does not contain DWARF symbols, or any other means to recover information so it's most likely impossible to see what's changed for PS4/5 in specific.
res.pak
Mostly unremarkable, you can unpack this with any standard PAKTool or its forks.