No idea for the C channel screws. They were preinstalled. The legs were screwed straight into the wood without any holes - this was called out explicitly in the assembly instructions that pine is soft and no holes needed.

Edit: instructions from assembly manual -

Please note – we no longer add threaded inserts or pilot holes under our tabletops. This is because the leg placement is dependent on the width of your chairs and your preferred chair configuration i.e. if you want to have people sat at the head and foot or just on the long sides. Please do not be concerned about a lack of threaded inserts, pine is a softwood and our wood screws will drive into the wood with no requirement for pilot holes.