For anyone old enough for this to make sense but who hasn’t seen a Game and Watch game: they worked just like the cheap Tiger Electronics LCD games you could get.
Just like a simple digital watch, everything that could be on the screen was designed upfront and then the individual bits could just be shown or hidden.
Nintendo eventually made three collections of the Game and Watch games for the GB. Each collection was a ton of fun with each game available in classic (faithful) or updated (graphics replaced with sprites, same gameplay) modes.