Robocraft has been free to play on Steam for years now. The game allows you to craft robots and then take them into battle against one another, which is a pretty simple premise. For those who are more into the building than fighting, you can create any kind of robot you like, from a hovercraft with tesla coils on the front as a battering ram, to a dinosaur in a top hat. For those into the fighting, you can buy pre-made robots to take into battle.
With Robocraft coming to Xbox One in 4K as Robocraft Infinity, there will be a decent number of changes to how things work in the game. It won't be a free-to-play title, it's flashier, you pay upfront, and the way you progress through the game is different to account for the fact that it isn't f2p.
That said, having played a bit of both versions of the game, the Xbox One port is going to be pretty much the exact same game as is on Steam. Just with a slightly different twist in terms of what you pay for. Considering Freejam is bringing the game to console for the first time, it does make a decent amount of sense that they'd go the traditional console route of paying for the base game rather than having it be a free-download title like on Steam.
And because of the difference in payscale and the inherent advantage of keyboard and mouse over gamepad controls, Robocraft and Robocraft Infinity will not have cross-platform play.
If you missed the E3 Xbox One announcement trailer, here you go.