Bike manufacturers generally equip their bicycles with non-proprietary common components. The components I have in mind are the drivetrain components (crankset, chain, derailleurs, shifters, hydraulic hoses and shift or brake cables). With press fit bottom brackets, some BB standards are unique to only one manufacturer (e.g. Trek's BB90, albeit this is being phased out, Cervelo's BBRight, which they seem to want to stay with). They aren't quite proprietary, in that many aftermarket component manufacturers will make BBs for those standards. In the past, all forks weren't proprietary, but this has changed substantially, and it is harder to find aftermarket off the shelf replacements.
This is changing for some components at the higher end for performance-oriented road and gravel bikes. For drop bar bikes, aerodynamic considerations are leading to hidden cables, wires, and hoses. Many high-end bikes may have proprietary handlebars and seatposts as a result. Even among D-shaped seatposts, there is no one standard geometry, so each D-shaped seatpost would be proprietary to the manufacturer and often to a bike model as well.
When looking at the specs at the Giant Roam 1 2021, I noticed some inconsistencies in the spec sheet: the rated capacity of the derailleur is not sufficient for the proposed drivetrain (46/30 in the front, 11/42 in the rear -> 48T vs 41T for the proposed RD-M5120-SGS) ...
Addressing this, I haven't verified your capacity calculations, as the Giant US website didn't specify the derailleur model and I'm not familiar with MTB drivetrains. Shimano's official specs are known to be conservative, so Giant might have exceeded them by a bit. I would be slightly surprised if they did this, but I would assume that Giant tested the setup. (NB: note that I said "assume"; I don't work in the bike industry and I don't know this for a fact.)
Wheels are a bit more complex. The spokes and hubs of wheels sold as OEM on bikes are usually not proprietary. I can name some exceptions, e.g. Campagnolo and its captive wheel brand Fulcrum use proprietary spokes, hubs, and rims, Cadex (a captive brand to Giant) has some proprietary wheelsets, etc. Higher end rims may be proprietary to a company, although you can replace a broken spoke with any spoke of the correct length (barring some unusual construction like carbon spokes (e.g. Cadex, Hunt), aluminum spokes (e.g. i9, some Mavic wheels in the past). The hubs on these OEM wheels are often proprietary as well, although replacement cartridge bearings are almost always non-proprietary. (For loose ball bearings, steel balls aren't proprietary, but the races are a different story, as would be ceramic ball systems like Campagnolo uses in some wheels.) Barring some unusual spoke count (e.g. some Campagnolo wheels use a triplet lacing system, and some have odd numbers of holes), you could lace an off the shelf rim to a manufacturer's hub if you wanted to and you were willing to put up with the aesthetic mismatch.
Derailer hangars are proprietary to each model of bicycle, but Wheels Manufacturing does stock aftermarket hangars for most bikes. Thru axles are also proprietary to each bicycle model, but there are also aftermarket replacements (e.g. Robert Axle Project).
