Build a Starter PC for a 15yo.

But instead of choosing for cheap components and get a low tier GPU,
I recommended to make use of the 5600G's VEGA 7 iGPU to play the games you like now (Roblox, Minecraft, Fortnite and Rocket League)
and later when prices get somewhat near to MSRP put in a nice GPU (if needed at all) or upgrade it later when you'd like to play more demanding games.