RetroPie doesn't boot into the Vice emulator by default, did you modify the auto-start sequence ?
I have software/some games, how do I load/run my games on the Pi400 C64??
You can copy the game files (floppy/cassete images) in the c64 rom folder and you can then load them from the Vice menu. Normally, if you start in the EmulationStation front-end, they would be listed and you can just start each one from the list with vice/lr-vice.
Do I put all my games onto a Flash drive and plug that into the Pi400?
Or do I put them on the SD card?