but .. it doesn't really explain it. Like it doesn't work the other direction.

The reason fusion and fission both "work" is because of the fuel. Iron (Fe) is the breaking point. If you can get things lighter than Iron to fuse, you get energy. If you can get things heavier than Iron to break apart, you get energy.

Flip it around and both operations take energy. Hydrogen+2/+3 and Uranium-235 are the easiest fuels for us to use for both of those operations.