I would like to buy a good gaming PC up to 650 euro, which I can easily play Fortnite, Overwatch or Black ops 4, you would have suggestions which are good
I would assemble one myself. That probably comes out the cheapest or because you can get the best hardware for the price.
https://www.computerbase.de/forum/threads/der-ideale-gaming-pc-spiele-pc-selbst-zusammenstellen.215394/
Read that.
Here is a good setup:
Motherboard: Asrock B450 Pro4
CPU: AMD Ryzen 5 2600 or 3600
CPU cooler: Boxed cooler if you do not want to overclock otherwise be quiet Dark Rock 4
RAM: 16 GB RAM z.b. From Corsair
Power supply: bequiet PurePower 11 500 watts
Graphics card: AMD Radeon RX 580