I don't know Xidax but I've bought from a similar (if larger) laptop-building business called Clevo/Sager.
These companies do laptops, they have access to all sorts of spare laptop parts, they might even make a few custom parts, they know how it all fits together, they know how to pack the most tech into a constrained form factor. An end-user has few options for buying a laptop motherboard or keyboard or chassis plastic, outside of normal consumer channels, small quantities (like just enough pieces to build one unit) cost more to obtain and it's sometimes difficult to confirm compatibility or fit before the parts all arrive. If you go this route you'll be building yourself a major project and you'll pay premiums on everything and you may or may not be able to get everything you want - but it's not impossible and people can DIY custom laptops quite successfully (that's where all these laptop companies start up, lol). But it's hardly worth the effort if you're planning on building a single unit or doing "casual" customization/upgrades.
If you use an existing laptop as a base platform then you're locked into whatever happens to fit inside it. This might lock you into some limited hardware choices, it often locks you into whatever upgrade path the manufacturer offers. It might be easy to swap motherboards between G751 models, but it would be difficult to find a compatible Z170 motherboard if Asus doesn't happen to sell one. You're also locked into whatever GPU upgrade path (or lack of it) they might offer, even things like the mPCIe form factor are not yet really standardized across all manufacturers.
You can upgrade the drives and memory on almost any laptop, unless it's already populated with the best it can support or the manufacturer chose to install fewer slots/ports/connectors on the motherboard than a full-sized chipset could run. You can often upgrade your battery with something newer (and more energy-dense) or something bigger. If you're unafraid of some soldering then you can completely upgrade your charging circuitry or internally splice USB ports into hardware features (like WiFi, GPS, whatever) your laptop doesn't natively support.
If you want a big project and the ultimate laptop then it's a good option. If you want less headache then go with a laptop company. You pay a premium either way. If you're targeting a top-tier extreme-performance model then you'll get better bang for the buck buying a premade business/gaming laptop and you'll get far better bang for the buck building a desktop. Research Intel/AMD mobile chipsets and procs (which are usually soldered together as a pair these days) then narrow your choices down to the laptop companies which offer the platform you want - how you obtain your chassis, plastics, keyboard, etc (and how you fit it all together) is another problem entirely. Be prepared to (re)manufacture your own heatpipes and rads and ducts from scratch, lol.
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