So To any napoleonic enthusiast explain this, how were there lines formed? A french regiment would consist of some fusiliers at the center with detachments of grenadiers and voltigeurs at the side. Why? Why didn't they create completely different regiments, which they did, rather than mix a line up?

a little diagram so you can understand

Grenadier/Grenadier/Fusilier/Fusilier/Fusilier/Volt/Volt. one line of infantry.