For example, if I play Fortnite games, can I buy items there with Google Play and are they then also on the linked account of Epic Games or can I not buy currency (V-Bucks) with Google Play?
If so, what other games are there that work or could work?
The purchased in-app content is linked to the game status and the game status is linked to the EpicGames account at Fortnite. The content is then also available on the other platforms.
Since Fortnite is no longer available in the Play Store, it becomes difficult with the in-app via the Play Store. You would have to use the alternative payment methods from EpicGames.