There were game engines in the 90s -- Quake Engine, Build Engine, Unreal Engine.. to name some of the popular ones.
You also had GoldSrc, a modified Quake Engine for Half-Life 1.
Many companies created their own and was not available outside the business. Dark Engine used in the Thief:Dark Project games is an example. Think of companies like Nintendo having their own engine which powered games like Mario 64 and, I believe, Ocarina of Time was a modified version of that.
The "off the shelf" engines available at that time (like Unreal or Quake engine) might have been decent for certain type of games.
Today, you could say many games could be done with Unity, Godot, or Unreal. There are still companies today using their own.
You also had GoldSrc, a modified Quake Engine for Half-Life 1.
Many companies created their own and was not available outside the business. Dark Engine used in the Thief:Dark Project games is an example. Think of companies like Nintendo having their own engine which powered games like Mario 64 and, I believe, Ocarina of Time was a modified version of that.
The "off the shelf" engines available at that time (like Unreal or Quake engine) might have been decent for certain type of games.
Today, you could say many games could be done with Unity, Godot, or Unreal. There are still companies today using their own.