Depends on the company. There's a big difference between a boutique pre-built and 3rd party pre-built. The boutiques like Origin or MainGear or Falcon Northwest all use aftermarket parts you specify on the build page. So you'll get an authentic asus rog board, Evga gpu, Seasonic psu etc. They do all the labor and you can get extras like braided colored wiring, mild OC, liquid cooling, rgb setups etc. Even a fully customized automotive painted and etched design case. You send a puc of what you want etched in the tempered glass.
The bad part is that all costs. Almost always far more than building it yourself. Wasn't long ago when nobody really ever recommended the boutiques since it was the same parts at @ 1.5x the cost to do it yourself. And everybody had a minimal budget and maximum wants.
3rd party is Dell. Lenovo. Those are rarely upgradable, minimalistic, basically B Movie pc's. They work great for an office, but aren't really designed for gaming, and those that are are subject to proprietary limitations. Oddball size psus that aren't atx standard. Mains power is 14 pin, not 24, so no 10900k. The windows is tied to the mobo, so no easy or cheap swaps. Bloat-ware. No Z490 chipsets. Bios is limited to select few cpus, so no real cpu upgrade etc etc.
The best part about boutiques right now is that they actually can get gpus. You might pay $2k for a $1200 pc, but you'll get a 3070 and not be buying a gpu for $1800 from a scalper after spending $1k to build a decent platform. End result is cheaper, guaranteed, warranted. And usually better looking than some pc's I've seen ppl put together.