The Steam Cloud syncing with your Nintendo Switch device is a one-way manual sync that brings over all of the achievements and unlocked maps from your Steam account over to your Nintendo Switch so you can pick up right where you left off. It is not a continuous live sync between the two copies of the game, and you cannot bring the progress from Nintendo Switch back over to Steam.
To enact the Steam Cloud sync with Nintendo Switch, head to your Nintendo Switch device and navigate to the Mini Motorways Options menu. From there, there is a new Cross-Save panel. You can import your score records and unlocked maps from your Steam account onto your Switch profile by following a few steps. First, you need to sign in to your Steam account through the Nintendo Switch web browser, and grant Mini Motorways access to your Steam Cloud data. This is all done through a secure process managed entirely by Valve, meaning that you never give us your Steam account password. Once authorised, the game will read your Steam Cloud data and merge it with your Nintendo Switch profile. Any maps you have unlocked on Steam will be unlocked on your Nintendo Switch profile; however, any in-progress games will not be imported, and any unlocks on your Nintendo Switch device won’t sync back to Steam.