Pain.
I seem to recall some extras at the end of the novel, but yeah, the actual business was pretty much implied, not graphically spelled out. Still, there were some pretty sexy moments. It was a far cry from another one of her works, Scumbag System, where these scenes focused on pain, not pleasure. Mo Xiang Tong Xiu takes a very ambivalent approach to male coupling, and some of it is harsh.
It is implied that they've definitely had sex together, multiple times. There are some steamy moments that don't quite go all the way. They are definitely in love with each other and passionate about each other. They are definitely sexually turned on by each other. They definitely care about each other as friends and partners, too. It has been awhile since I've read the novel, and I can't remember any explicit descriptions of sex between the two of them per se, although there is some pretty kinky stuff going on because they are in a magical world and magic informs every part of their lives. Plus, Hua Cheng has carried a torch for this Crown Prince almost since the very beginning. This is not unrequited love. This is very much reciprocated and passionate.

Was talking to a friend about yaoi and then I thought about telling her about tgcf like "you should read this. It's beautiful. imagine the sexy scenes with that art style"
Tiny spoiler:
But then I remembered there aren't really sexy scenes and it made me realize how the author made those two the sweetest and kinkiest couple of all her novels but decided to not show us and I just fell into depression with that realization all over again