Can you build a laptop

not from scratch like a desktop, basically you find a seller for the chassis that has the motherboard, GPU and a few other bits, and can buy the parts seperate. It can be done but really shouldn't be. it's not worth the money and companies who offer custom configs are actually cheaper than doing it yourself.
plus finding the parts all in one place is really difficult and there's a bit more knowledge in compatibility of parts than building a desktop.
hope this answers your question