The truth is I never build my own PC. It's either a friend who built it for me or I use a laptop right now. I want to try this game as a way to learn to build PC IRL but the price tag for this game is kinda steep especially with so many paid DLCs. Is this game realistic or more like a sim?

If you never built, sure it will help you.
But, beware, the reality can be uterly bothering when there is a problem: Building a pc is not a game.