Collection is a compilation of games packed to Unity engine. I don't know if anyone was digging deeper than him:
https://gbatemp.net/threads/might-i...ange-roms-in-sega-mega-drive-classics.525661/
We have now better tools to utilize Unity games on Switch, so who knows if it's now possible. I don't know specification of sega mega drive roms (size of cartridge, etc - edit: ok. 4MB, sometimes 5MB), so I don't know what to check.
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So from what I checked probably games are stored as "txt" in resources.assets
This is screen from g0008.txt
They have sizes 128, 256, 512, 640, 768, 1024, 1536, 2048, 3072, 4096 KiB
Idk if they are clean roms or edited
You can use f.e. UnityEx to swap files
Remapping will be harder to do as this will probably require serializing assets and extracting data from monobehaviour