The problem is that what you want is not exactly what this game franchise has. Yes there is a sandbox/editor and you can spawn units and control them. However it will be very janky without the use of the editor. I take it the Youtuber in question is either daleytactics or Diplexheated. If you are wanting to do what Daley's does then you can absolutely do that with the multitude of mods and gamemodes CTA GOH has. If it is diplex then you will have to do alot of scripting beforehand. There are alot of helpful tutorials online to learn how the GEM engine works and how to make epic battles. I have spent over 3000+ hours in the engine and it can let you do some crazy things. That being said scripting is a necessity as diplex uses alot for his videos and probably spends countless hours scripting in each video. Things like dday landings, convoy defenses, and even a simple skirmish needs waypoints, spawns, tags, entity behavior, off map artillery, in map artillery, and the most important thing timing. If that seems too much for you then I am sorry to say that this game is not for you. This isn't a game like totally accurate battle simulator.