Will this work with my tonneau cover or does it replace it?

This kit is a complete cargo box and roll bar system — it functions as your tonneau cover. You won't need a separate cover.

Does this need any welding or drilling to install?

It mounts at factory bed points, so no drilling. Bolting procedure depends on your truck's attachment configuration — review the install guide before starting.

Can the cargo box be opened from the bed or only from outside?

Access method isn't specified in the product details — verify with the install documentation or reach out with your truck's year and cab configuration.