The antenna is likely a custom part just like it would be in a laptop. You *may* be able to find one on Ebay that someone pulled out of a broken system.
You can see on the official page that the wiring goes into the top of the chassis:https://rog.asus.com/articles/gaming-desktop-pcs/rog-huracan-g21-upgradeable-compact-gaming-desktop/
Alternatively, you could just buy an add-in m.2 wifi/bluetooth card that comes with its own external antenna and follow the directions on how to mount it for optimal reception.
Easiest of all would be a USB wifi adapter.
A bus station is where a bus stops. A train station is where a train stops. On my desk, I have a work station…