What 𓆩 and 𓆪 really are
The angel wings 𓆩 and 𓆪 are Egyptian hieroglyphs, signs L006 and L006A in Unicode's Egyptian Hieroglyphs block, which was added in Unicode 5.2 in 2009. The two signs are mirror images of each other. Placed side by side, their feathery curves look like a pair of wings, and that is how they became the internet's favorite angel wings. In Egyptian writing they have nothing to do with angels; the wing reading is purely an internet invention.
Anything you put between them gets wings: 𓆩♡𓆪 is a winged heart, 𓆩✧𓆪 a winged sparkle, and 𓆩 Name 𓆪 frames your name. Leave out the spaces to make the wings hug the symbol.
Other wing pairs
For a bigger, more dramatic look, gamers use ꧁ and ꧂, ornamental marks from the Javanese script, as in ꧁ Name ꧂. For small, cute wings, use ʚ and ɞ (see wings symbols and butterfly symbols). The Odia letter ଘ makes the wing in flying angel faces like ଘ(੭ˊᵕˋ)੭.
In emoji, there is 🪽 WING, added in 2022, plus 👼 baby angel and 😇 smiling face with halo. More of those are on the angel symbols page.
Why the wings sometimes show as boxes
Because 𓆩 and 𓆪 are hieroglyphs, the device showing them needs a font that covers Egyptian hieroglyphs. Current iPhones, Android phones, Windows PCs and Macs include one, but older phones and some apps do not, and show empty boxes instead.
If your wings disappear, try ʚ and ɞ, which almost every font includes, or the larger ꧁ ꧂. A quick test is to send the symbol to yourself and open it on a second device.