Do you ever think there would be hybrids (I can't think of anything else to call them) between elves and humans?
Okay this is slightly complicated so*breaks fingers* prepare yourself
Elves and humans are cannonically different species, as far as I'm aware. Now, according to this article, "Most evolutionary biologists distinguish one species from another based on reproductivity: members of different species either won't or can't mate with one another, or, if they do, the resulting offspring are often sterile, unviable, or suffer some other sort of reduced fitness."
So, bearing that in mind, it would make sense if elves and humans usually do not create offspring together. Maybe there's something about the two of them that just feels off and unattractive to the other specie. Maybe humans are just... so messy, to elves. Maybe humans are wrecks, messes, or maybe we stink. Maybe elves find us gross, in some unforgivable way that they never talk about. Like, sure, they can respect us as a species, but they're not going to marry us.
Likewise, humans presumably find something off with elves, as well. If you've ever read The Lunar Chronicles(10/10 would recommend if you have not) you know that perfection is off-putting to humans. Like, perfectly symmetrical faces, incoherent beauty, over the top perfection. When it reaches a certain point, we don't like that so much anymore. There needs to be some flaw, something human about the person to make us attracted to them. Moreover, we look for traits we can find in ourselves in potential partners, as well as places to fill in the gaps of our own flaws.
"Well, hold on," You say, mildly concerned, "Sophie describes elves as beautiful! Like, crazy attractive, all the time! I would totally be attracted to an elf."
Well, hang on a second. For all her humanoid tendencies and behaviors, at the end of the day, Sophie is an elf. She has the mind of a human, but the biology of an elf. And, considering that what you find attractive has quite a bit to do with biology(most neurotypical people wouldn't find a different species attractive in any sort of mating kind of way), she is naturally going to be inclined to find elves more attractive than humans. Because she is one.
Anyways, I'm sure it's happened, that elves will fall in love with humans, or vice versa, and perhaps they got married and had children. Lots of things happen. However, according to biology, and the fact that elves and humans have different DNA, the children will probably have mild birth defects, or what one of the two species would consider a birth defect, such as being talentless, or never aging but growing old internally(like that one super sad Steve Rogers fanfic i read once oh my stars), or even aging normally but never dying. Maybe elf/human children have heterochromatic eyes, one that has to be blue because of the elf genetics and one that has to be brown through human genetics. But, if nothing else, most likely these kiddos will suffer from infertility.
Anyways, I'm sure these kids have a huge personality crisis going up, and have to pick one world to live in, despite how their parents seem to think that they don't have to. If they age as elves do, they'll have to watch their human parent and entire mass of human relatives die in front of them, and possibly some of their siblings who age as humans do. And, especially if they're exhibit more elf-leaning traits, the grief will be really hard on them, as will it be on their elf parent.
Also, if they're more elf-leaning, this brings up the classic problem of talents and matchmaking. They could have a small talent, or some small ability. Anything powerful or flashy probably wouldn't get passed on. However, they're definitely not eligible for matchmaking, and probably would have to live their life as a talentless if they cared to.
"Hold up," you say, "What if they decide to enter into the human world! They wouldn't have to worry about matchmaking or abilities then!"
I present to you, the immortals. The actors who never age, the ones who share names with people who supposedly died hundred of years ago. I present to you, those with clear abilities that are so subtle they wouldn't effect an iota of the elvin world. Those who show up in your dreams to offer advice but then vanish and you can still remember their faces. Those who have such an understanding of mechanics that they can slap something and it seems to almost fix itself. Those who understand what animals want and need like they understand language. Those who can understand exactly what you're feeling, all the time. I offer you those whose voices are timeless and speak of thousands of years of knowledge. I offer you those who say, beleagueredly, "I don't want to live forever." I present to you the old woman who lived down the street who you swear has to know magic because once you visited her house and it smelled like burning stars and whispered memories. The old woman at the end of the street gets older and older every year. But you don't think it's possible for her to die. Your parents remember her being that old and living in that same house. So do your grandparents.
I feel like that's where the elf/human hybrids would take refuge. Living in safety among humans where they would be ostracized among elves. It's not that the human world is better than the elven world, it's just a little more comfortable for those who are different.