Can I redeem a code online on Xbox One in the web browser, not on the console and then receive the content without starting the game?
So for example: I enter a code for a game and start the game with the same account on PC / PS4 etc. And have credited the content, or do I have to open the game on Xbox first until it works?
Yes, if you redeem a code for a game, it will be credited to the game as soon as you open it (or as soon as there's an internet connection and the console is switched on).
Microsoft.com/redeem
this is possible on this website
That means I can redeem the code for Xbox Online and then start on PS4 and I have everything?
No, you can't redeem a code for Xbox and then have the content on PS4 unless the game is crossplay and you play with the same account in the game on both consoles.
Otherwise it doesn't work because the online services of Xbox (Microsoft) and PlayStation (Sony) are different and have nothing to do with each other.
At Fortnite? (Is crossplay), should go then, right?
If you use the same Epic-Games account, then yes. It must be the same savegame, i.e. The same account at the game.
If you have not yet connected your character or your gaming account to the Epic-Games account, you should do so beforehand. That's the only way.
