I'm using Blender 2.93, and I've seen so many YouTubers just apply a mirror modifier and they're able to create two objects. The only way I know how to use it in this version is to add an empty object like a hook and use it as the mirror object. But I've started sculpting recently and I'd like to do it the way it's done in this video: https://youtu.be/Lso8KitmchY. I've heard that you can mirror two objects using the world origin but I tried that and it didn't work. Either I did it wrong or I still don't fully understand how to do it. Any ideas, descriptions, or alternative options would be appreciated.
The Mirror modifier in Blender works based off the objects origin. If you want to mirror the object as a whole you will need to go into Edit Mode and move the object off of it's origin point. Then when you use the Mirror modifier it will mirror the object on the chosen axis based on how far from the origin it is.
There's no such thing as using the world origin in a modifier. Modifiers either use the current object's origin (which you can set to anywhere you want) or another object's origin (which you can also move anywhere you want.
In either case you can do it so you effectively have your modifier acting from the world origin.