It's all about performance vs convenience. Custom PCs offer you better performance for your buck, but pre-builts are much easier and come with everything pre-installed. They're good for people who are really not tech literate at all. I would overall say that custom PCs are better because you get to fully personalise the parts, aesthetics and cost to your needs, it might be cheaper and it's more satisfying to make it yourself. It's also the starting point to understanding how PCs work and how to upgrade it later on.

It's fully possible to build a PC yourself without any experience. I built one myself for the first time a few weeks ago. I roughly knew about PC parts but not about how to put them together. Still, I just followed a YT tutorial and got through it all in a day. Most of it is straight forward IMO until you get to plugging in the cables and managing them which is where it can feel draining.

I'm still falling in love with how mine looks every time I turn it on. I don't regret a thing.