The main versions of Apex Legends are listed as using the Source engine. Respawn Entertainment built the game on a heavily modified version of Valve’s Source technology rather than treating it as a completely new engine.
That choice connected Apex Legends to Respawn’s earlier work. Source had also been used for Titanfall 2, but Apex required substantial modifications for its large battle-royale maps, longer sightlines, and demanding multiplayer environments. The engine was pushed toward capabilities beyond the traditional close-range level design associated with many older Source games.
The answer can be confusing because Apex Legends has not used exactly the same technology across every release. The main PC and console versions use modified Source, while the mobile edition was listed separately with Unreal Engine 4. “Frostbite,” “Unity,” and id Tech are all prominent game engines, but none is the engine listed for the main versions of Apex Legends.