Serious question: is English your first language?
There's no ambiguity here, the hobby is called model building.
But they selecting the individual components, buying them from different vendors, then assembling them to the final thing.
You can "build" a PC or a bicycle without creating any custom part, but the end result will be unique. A keyboard has less parts(board, switches, keycaps and maybe a controller) but I think it still count as built.
If you order everything as part of one kit, like the furniture from Ikea, then yes, you just assembling it together.
I assembled many model kits and it never occurred to call myself a builder. Model kit builder concieve the kits and or create parts out of raw material, epoxy putty and/or 3d printing.
Similarly a bicycle builder will solder/bond individual tubes to build custom frame and fork, possibly custom stem and/or racks. A brand that source frame builts in Asia will but mount the ready made part will say "assembled in <insert the country>.
Maybe I am picky with semantics but it strikes me as arrangant to call ourself builders when we just pick up and assemble parts.