Suzanne Clarke wrote:

We are replacing some decking and have the boards delivered. The
previous deck was installed by the builder's chippie about 12 years
ago. We have started taking the old boards off. The screws, which
were invisible before, start half way into the plank and then screw
into the base. How on earth is that done?
It's a bit like "secret nailing" but done with screws diagonally through
the tongue/groove

https://www.mapleplastics.co.uk/camo-hidden-deck-fasteners

Or there are some clip fasteners that don't actually screw into the
boards, but clip into the groove.