After looking more closely at the photos, it looks like the specific hubs/wheels in question are the Prime Comp Alloy Clincher Disc Road Wheelset seen here. These have Prime RD020 Disc Hubs - front and rear. Looking closely, it doesn't seem as though these are rebranded novatec hubs, but they do seem to utilize a very similar end cap design.
For the front, it looks as though the end caps pop in:
for thru axles, the axle is separate and should be included with your frame, the end caps simply provide the mating surface to the frame/axle.
15mm thru - upper surface mates to fork
12mm thru - I believe the top part shown here is the interior portion of the cap
These caps are something that simply 'fills the gap' between an oversize bearing inside the hub and the desired axle.
On these particular hubs, the rear seems to operate somewhat differently. The drive side end cap appears to thread onto an oversize axle (to set preload?). I believe that the axle itself functions as the mating surface for the non drive side of the hub.
Hub in QR 130x10 configuration with all included parts
142x12 axle and drive side end cap
Excuse the question marks, but Prime is pretty sparse on details. It appears to be very similar to Novatec's configuration, which is shown in full here and is where I drew some inferences:
refer to the lower half for the rear hub, although Prime never pictures any non drive side end caps. I do not believe this particular front hub works as pictured here.
Many companies are doing something similar now, all with their own little spin of course. They make a hub with oversized bearings that create some physical limit as to the maximum axle size, and then use caps to effectively shim the interior diameter and determine the final spacing of the hub.
SRAM recently introduced their roam/rise/rail wheels which utilize end caps that pop in. SRAM also has some proprietary end caps for the front called torque caps that increase surface contact for increased stiffness (they claim).
clockwise: 9mm drive side, 15mm drive side, 9mm non drive, 15mm non drive
A nice blog post here that describes the process of changing the hub to a different axle standard. A few photos from the post:
I'm having a very hard time finding more useful images, but some more brands that I know employ end caps (feel free to edit):
- DT Swiss
- e13/The Hive
- Salsa (made by Formula)
- Industry 9 (i9)