What consoles is Fortnite available on?
Fortnite is available on PS4, Xbox One, PC, Nintendo Switch (iPhone and Android devices). As a general rule (and there are some exceptions here with PS4 that will hopefully be addressed soon), you can play Fortnite with the same epic account on multiple consoles. You can login to your Epic account on your iPhone and play while you are out and about, and later log back in and play with your same Epic account on your Xbox One.
Fortnite on the Nintendo Switch.
Can I play Fortnite with friends on a different console?
Yes, you can. Fortnite cross-play allows you to play in a lobby with other friends playing on different platforms. Note that if you are playing cross-play your lobby will also contain other players playing on mixed platforms. Keep this in mind if you are playing on mobile, you may have a hard time keeping up with players that are playing on consoles.
Is the gameplay on all Fortnite platforms the same?
Yes, Epic Games has done an excellent job of providing the same great Fortnite experience on all platforms. The only thing that varies is the control mechanisms. Mobile players have a unique control setup, but the gameplay mechanics are the same whether you are on PC, Xbox One, PS4, Nintendo Switch, or mobile. The graphics may vary a bit and you may notice some lag on some systems, particularily the Nintendo Switch as the graphics processing power on it is not as great as what you'd expect from an Xbox One or PS4.
Left: Fortnite graphics on iPhone X - Right: Fortnite graphics on Xbox One