Thankfully the world has caught up a lot in the world of prototyping and sadly devolved a little in the world of specifications. Tools such as unity, coupled with something like probuilder and other free or reasonably cheap addons can make designing a prototype of your game a doddle!
A Valve fanboy of my time the community would often put up prototypes using Valve's "dev" textures for trying the feel of the sdk game map to play through. The idea of prototyping in games is not new but the idea is still the same and highly worthwhile. Unlike valve maps you'll mostly likely want custom components that don't exist in code but many features will exist for you to drop in a fairly accurate replica to play around with.
Game jams have become their own thing of late but ultimately! These little mini projects made in a seriously stressful timeline are just prototypes for games that may never see the light of day as commercial products. Games such as super hot started life as a game jam, but take note how long that took to become a commercial project. A prototype is a great start to a game but requires a lot more to make a final polished product you, as a developer, can feel comfortable selling and often require specification and scoping to really make a decent product, something i'll get to in another post.
Useful Links:
Probuilder Tools
