Rear derailleur doesn't go to smallest cog

No mechanical part is perfect in real life. There's a tolerance for everything, which means that your two identical model hubs may actually be slightly different dimensions. We are talking about a few tenths of a millimeter at most, but it only takes that much misalignment for your shifting to start feeling off.

Make sure your cassette is properly fastened.

You've unscrewed the barrel adjuster too much, which is pulling the cable too tightly and not allowing the derailleur to access the smallest cog. You should search up "how to index bike gears" to learn how to adjust the derailleur properly. You don't see any exaggerated movement because again, it only takes a fraction of a millimeter to make the shifting start to degrade. Even with lower-end derailleurs, each "click" that you feel of the barrel adjuster is 0.1mm, which is a tiny distance. Your high-quality R8000 derailleur has an even finer adjustment, so don't be surprised if you can't discern any movement. Don't worry, it did indeed move.

Speaking of barrel adjusters, they're basically a hollow bolt. Screwing them in decreases the cable tension, while screwing them out increases it (righty tighty lefty loosey). The outer sleeve thingy is both a cosmetic cover and also gives you the clicks you feel. When you screw the adjuster bolt in or out, the sleeve slides down thanks to the spring and hides the unsightly gap you're creating, adding stability to the assembly as well. There's little teeth on the sleeve which mesh with little teeth on the derailleur body itself, resulting in clicks.