The roost is for sleeping on.
CHickens instinctually prefer to sleep in the highest spot available. So as long as you put your roost higher than any other "competing attractions", and as long as there is nothing actively repellant about your roosts (too thin, too slippery, right next to cold wind in winter, insufficient room for chickens to get up there) they will naturally gravitate to the right place to sleep. Er, once you have trained them to roost at least (some chickens do not initially figure it out on their own).
Since the roost is some sort of 2x4 or wide-ish tree branch, there is no issue of putting anything on it. It's just *there*, for them to sit on and snooze
Nestboxes are for egglaying. You want something cushy inside so that when a hen lays an egg and then stomps around getting out of the box, or another hen comes in to lay *her* egg and stomps all around the nestbox, there is sufficient padding that the egg doesn't get broken. Fluffy wood shavings like you might use as litter on the coop floor will work well; as will straw, or hay, or various other things. You have to pick out poo periodically (chickens occasionally evacuate the wrong thing from their cloaca when they *meant* it to be an egg
) and occasionally replace all of it.
Chickens will often seek out your nestboxes on their own, when they want to lay, just because they like somewhere secluded and cushy for the event; but it can help to put unbreakable fake eggs in there ("nest eggs") to give chickens the idea of what the nestbox is for. Since chickens have a natural tendency to want to add to an existing clutch rather than go lay their egg somewhere else, this really does help quite a lot of the time. Once the chickens are well-confirmed in using the box, you can remove the nestegg as it will have done its job.
Good luck, have fun,
Pat
