Poop board: yes, a board under the roosts. Many of them have a board around the edge, so it's like a tray, and then a layer of sand. People scoop the poop up each day, similar to how you'd clean a cat's litter box.
If you prefer to spend a few minutes each day to remove as much poop as possible, it's a great invention.
If you prefer to let the poop pile up or get mixed with the bedding, then clean it all out a few times a year (to the compost pile or garden), then a poop board isn't the thing for you.
Sand: appears to work great in dry climates, but some people report it gets stinky when wet.
I have no personal experience with the horse pellets.
For flooring, you might look into "deep litter" and "deep bedding." Both involve piling plenty of stuff into the coop (or run) and not cleaning it out very often. One rots/decomposes like a compost pile, the other stays dry enough that it does not rot. Some people distinguish the two, and others mix up which name goes with which--personally, I can't keep the names straight

Either way, the basic principal is to add sufficient dry matter to keep it from being a swampy mess, and provide sufficient ventilation. The chickens tend to keep scratching and mixing and re-arranging it, and find lots of little bugs and things to eat.
There are lots of "best" ways to manage chicken waste, so I cannot say what will work best with your climate and your own preferences