Don't want to write this by hand? GodCraft sets minecraft:unit_cube for you with a switch.Get GodCraftOverview
minecraft:unit_cube is a block component: it goes in the components object of a custom block. Draws the block as a plain full cube. This is an older option; minecraft:geometry with "minecraft:geometry.full_block" does the same.
Example
Inside the components object of BP/blocks/<name>.json (values are the defaults or placeholders):
BP/blocks/<name>.json
{
"minecraft:unit_cube": {}
}