Is it possible to build a laptop yourself like a desktop PC? If so, where do you buy the parts, where are tutorials, etc?
You can. But for the cost and effort, you can buy three finished laptops.
I would be interested. How much is that about (not great)?
The effort would be pretty big and if you have not even built a pc together, it will be quite difficult with the laptop
and the motherboards are custom-made, I do not know how you get to it
It's definitely possible, but you have a huge amount of time, high costs, and hard-to-find replacement parts!
Since the parts have to be smaller than a desktop PC, they are also more expensive.
For the price you can buy 2 finished laptops with the same performance I would say but if you enjoy it then just do it!