Is it really hard to make a car PC setup with raspberry pi sort of a unit and ship it with a really good user interface?
And is CarPlay is a paid service that I need to purchase if I want to build my own hardware?
So I'd recommend you just buy a reputable Carplay-capable unit that fits your budget.
A DIY CarPlay unit using a Pi will end up requiring a lot of research and will cost you more money than you'd spend just buying a pre-fab unit.
I am not aware of any easy path to DIY CarPlay.