A game engine is the software a studio builds a game on top of rather than from nothing. It draws the scene each frame, so it owns rendering: cameras, lighting, materials, the pipeline that turns a 3D model into pixels. It simulates how things move and collide, which is physics, and it plays and mixes sound. It gives designers a scripting layer, often a language simpler than the engine’s own, so gameplay can be written and changed without touching the renderer underneath it. It bundles editing tools so artists and designers can place objects, tune values and preview the result without a programmer in the loop, and it abstracts away the differences between a PC, a console and a phone so the same project can target more than one. None of this is specific to any one game; it’s the machinery every game needs, built once and reused.
Unreal Engine and Unity are both licensed, and both licenses are written to scale with a game’s revenue rather than charge every developer the same amount up front. Under Unreal Engine’s End User License Agreement, using the engine costs nothing until a product built with it directly earns more than $1,000,000 USD in lifetime gross revenue; past that point, a standard license owes Epic a royalty of 5% of further revenue from that product, which drops to 3.5% for a release that ships on the Epic Games Store no later than anywhere else. A separate seat-subscription requirement applies instead of royalties for some other uses of the engine, with exceptions for hobbyists, students, and anyone earning under $1,000,000 a year, as the agreement stood when we read it. Unity’s current pricing runs the other way: revenue and funding decide which plan a team is required to buy, rather than what it pays afterward. Unity Personal is free for a business under $200,000 in revenue or funding in the past 12 months; past that, Unity Pro is required, priced per seat; past $25,000,000 in annual revenue, Unity Enterprise is required instead, with custom pricing and source access. Both companies can and do change these terms, so the numbers here are what each company’s own page said on the date we cite it, not a permanent fact about either engine.
Godot is licensed differently in kind, not just in degree. It’s released under the MIT license, which lets anyone use it for any purpose, modify it, and distribute it, including commercially, without paying Godot’s makers anything or asking permission first. The only real obligation is keeping the license’s copyright notice attached when you redistribute the engine itself; a game built with Godot is the developer’s own work, licensed however the developer chooses, with no revenue share owed to the engine at all. That difference is why some studios choose it deliberately, and why “how much does it cost” and “what happens to my revenue at scale” are two separate questions worth asking about any engine before a project starts.
Plenty of studios use none of the three. A large studio with a long-running franchise, a specific technical need, or enough scale that even a small royalty adds up to real money, will sometimes build its own engine instead: this masters a company’s own pipeline instead of a vendor’s, avoids royalty or seat costs on every future title, and lets the tools track exactly what the studio’s games need rather than what a general-purpose engine offers everyone. The tradeoff is that the studio now owns every problem the engine has to solve, indefinitely, that Unreal, Unity and Godot’s communities and documentation would otherwise have solved once for everybody who licenses or forks them.