Bikes actually lend themselves well to online buying. They don't go stale, or melt. Are robust. Bikes brands are really frames with other stuff added on customized. Or bespoked. So buying yourself the components is just what a bike shop does. Assembling is no big deal if you have time. Brakes, tyres, bars, rims, tubes, mudguards,handlebars, gear wheels, shifters etc are usually all different brands.
But the tweaking and adjusting and servicing and getting special stuff on is best done by experts. Your local bike mechanics.