Super Mario 3D All Stars has leaked onto the Internet.
It appears all the games are emulated.
Galaxy and Sunshine run under a Wii and GameCube emulator named "hagi"(?) possibly made by NERD (Nintendo of Europe division).
Mario 64 is running under an N64 emulator. Dunno which.
Galaxy in particular is really interesting. It appears they recompiled the original code to run natively on the Switch CPU, but everything else (GPU/Audio) is running in the emulator.
Interesting trick!
Information on the N64 emulator:
Other minor tidbits...
The project codename might be "Stardust". There also might be a sub-codename for Galaxy - "Shigeru".
The menu that lets you choose games is made with LunchPack (engine used by Splatoon / Mario Maker / Animal Crossing).
Regarding other games found in the N64 emulator:
On SM3DAS development time:
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