When talking about humanoid flying beings, people often point out the necessary wingspan (it's a lot) but what's less discussed is the muscle structure necessary to make wings move. You can't just stick wings anywhere. They need a complete muscle system for them, front and back.
There are a few concept artists that have tackled this but I really, really like this one:
Skeletal and muscular exploration for a half bird - half human fantasy creature. I wanted it to be the most "realistic" and functional it co
Notice both wingspan (huge), muscles on the back AND the front, a second pair of pectorals basically. I actually think those are rather conservative for what it would be needed. An "angel" or "harpy" like humanoid would have a very prominent muscled chest and front besides the muscles on the back and of course the wings. And this will also result on an interesting disposition of the skeleton especially on the chest. This is this artist's representation but there are other ways, but note that those muscles need an "anchor" which is what birds have:
In my own biopunk setting, these modifications are so massive and strong that flying humans have not been fully accomplished yet, and de-novo attempts (that is, editing from before birth) are still very controversial. Most "cosmetic" modifications are relatively easy and can be done in adults: ears, tails, tegument like scales or feathers, the human body can be grafted with these with the technology of the setting. What cannot be done is just stick a pair of wings and expect to fly, the body adaptations to do this are massive. Similar issues come with changing really anything with regards of the overall bone and muscle organization of the body.
