The word pheromone does not mean what you believe it means.

Pheromones are chemicals released to convey information. They're used for chemical-based communication between creatures, particularly on social insects like ants. You're thinking of some kind of airborne sperm or an animal version of pollen. The pheromones would come into play to tell the "animal pollen" where to burrow into the skull to reach the egg and fertilize it (aka the thing releasing pheromones would be the egg), except that'd require the pheromones to somehow go from the brain to outside of the skull.

More or less related.. worldbuilding.stackexchange.com/questions/107103/…

I suspect you don't mean pheromones, & you don't appear to know what the word means (for all the reasons already stated in comments above ^), seems likely you just mean airborne which (as practically everyone, accept you it seems? already knows) many viruses are, here, [proffers an object] have a big fat [-] for lack of research, did you even Google the word pheromone b4 posting?

Pheromones are breathed in. Why would it be outside the realm of possiblity that an alien pheromone could contain reproductive properties that are activated after being breathed in? If the sense of smell is affected by pheromones, then they have to be breathed in. And why wouldn't the alien growth and DNA somehow be able to change a human's pheromone into alien pheromones after they've been changed?

There are limits on size of molecules for pheromones, if they are airborne like scent. Airborn particles can be bigger than molecules, but it defines speed of propagation, distance, and in a sense coverage area/properties, penetrative abilities. What you describe is not impossible, scent may have properties to trigger eggs to develop, but due limited mass it carry limited information, so it not reproduction, it a signal that inhibits(it may be a necessity as well, to be able to have hosts in the future) or encourages growth. Few sligth touches can make your setting better.

Thanks for the comments and feedback, I have modified the question to make more sense.

A lot of the issue here is that the simpler and easier a particle is to propagate, the less information and functionality it carries. A virus does an extremely simple task with relatively low efficiency and fidelity, made up for in vast numbers. Humans reproduce something of great complexity with high fidelity (including development, which is complex for humans) but with extremely low reproductive rates. Are your parasites more like a flesh-eating bacteria, or an intelligent life form? For what you're talking about, the infested human would need to be a bit of a more complex life cycle.

@DWKraus They are intelligent and I have edited the question to include the explanation of how their pollinated pheromones are able to infect people and form intelligent sentient alien brains that replaces our own brains. I hope it seems plausible..?

By all means and purposes, the chemicals contained in the pheromones don't ever reach the brain. They're sensed by chemoreceptors in the nose which then transmits electrical signals corresponding to what was sensed. If you feel like hearing my advice and want an airborne gamete to fertilize the eggs through the nose, it might be better to have something like what's seen in alien covenant: very small, dust-like organisms (or in here, alien "pollen") that actively force their way into the bloodstream through the nose/ears/softer bits of skin and then reach the brain.

Both pollen and pheromones relay information in a way, but the complexity level is completely different (one carries a simple message, the other usually carries about half the blueprints for an organism) but pollen isn't a pheromone and vice versa. If your reason for its use is mainly because you want a word that has an alien vibe to its gametes, I recommend using "spores" instead (especially since spores might contain the information for an ENTIRE being, granted the being is usually a fungus, but still).

@projectalex, yet drugs enter the bloodstream when they're snorted or inhaled through the nasal cavity, which is also where pheromones go when they're inhaled as well. This happens with drugs by contact as well. I don't see why there can't be an alien pheromone that could possibly enter our bloodstream and reach our brains in this way. I completely get what you're saying, but I like my set-up as is.

I also don't see a reason why the pheromone can't be a type of alien pollinated pheromone as a way for this species to keep their species continuing. Spores is an interesting touch and will consider that as well. I'm aware pheromones aren't pollen as well as pollen aren't pheromones here on Earth, but that's what fiction is for, to imagine if the impossible could just be possible.

ignoring the pheromone issue, this species reproduces by: (1) airborne broadcast spawning; (2) only using partial "sub-gametes"; (3) the sub-gametes accumulate on a suitable host brain until they can combine to form a gamete proper; (4) this happens twice (both sex gametes) in the same host; (5) once the gametes join, the parasite begins eating the host's brain; (6) eventually the host brain is entirely consumed; (7) the rest of the host body does not die but is taken over by the parasite; and (8) these parasites are sapient. That seems an implausible chain to me. Others may disagree.

I have edited the question to clarify that this alien species is a symbiotic species, not a parasitic species. Thanks for pointing that out to me, they do not eat the host body or harm the body after the brain has been replaced.

defining it as parasitic relationships was correct, symbiotic ones it when both sides get something useful for each other which is not the case with the brain-eating situation, at least as it looks like, so far, in your q. also pursuing that pheromone construction increases the complexity of situation drastically, and you pretty much can handwave at this point, not entirely impossible but problems and all that p to describe it in an answer is pretty much unrealistic. even if someone could produce such an answer on wb(unlikely), I guess, it would be hard to understand such an answer.

at this point, you asking for a sky.

Drugs are much, much simpler compounds than gametes. Their ability to pass through membranes to enter the bloodstream is also therefore much easier. You don't have an attack of cirrhosis when you inhale ragweed, because the pollen doesn't get into your bloodstream, so all you get are a runny nose and itchy eyes - places where the allergen comes in contact with the membrane, rather than penetrating it. If you want your alien gametes to be magic, make them magic, but I agree with the commenters here -asking for a "plausible" explanation is a bridge too far.