Which pc should I get where I can play like fortnite and play without lag?
You can play on almost any PC from $ 500 without lag.
I play on the MacBook
Assemble yourself or finished pc?
Ryzen 5 1600AF, 16GB 3000MHz RAM, 400W NT, B450 mainboard, RX 570 8GB
It should go really well on such a system, but it shouldn't be less, especially with the graphics card
Just in passing: Lags has to do with the Internet, what you mean is fluid, without frame drops or not jerky.