At this time, there is no cross-play or cross-progression between Xbox One and PC/Mac. If you purchase Fortnite on Xbox One, you will only be able to play on Xbox One with other Xbox One players.
Furthermore, playing Fortnite on Xbox One will lock your Epic Account to the Xbox One platform. This means that you will not be able to play Fortnite on any other platforms while logged into that same Epic Account.
If you still wish to play Fortnite on additional platforms, the only way to do so is to create a new Epic Account and purchase another copy of Fortnite for the desired platform.
Please Note: We strongly recommend that players who are locked to the Xbox One platform do NOT unlink their Xbox Live gamertag from their Epic Games account. Doing so will not affect the locked platform, as that is assigned on the Epic Account level.
We also recommend against purchasing additional PC copies of Fortnite on this same Epic Account, as this will not circumvent the locked platform.