This used to be a thing- I remember my father excitedly configuring a made-to-order laptop from ZipZoomFly[0] back in the day. I think that the market wasn’t kind to them though, the ecosystem about replaceable laptop parts never matured to the point where it was competitive with the proprietary designs, and standards constantly changed because of the form factor’ constraints, so the dream of just replacing a single part never materialized.
Closest thing to that dream now is the framework laptop, which does have replaceable parts.
Resellers of Clevo barebones offer a fair bit of flexibility to spec the system to order. It's not full freedom to mix and match, but still quite flexible. The price is that it is far less sleek, bulkier and heavier than most other laptops.
In Silicon Valley you can go to Central Computer, though they aren't a tiny neighborhood shop by any means.
For laptops you might buy a Linux laptop.