When we first begin writing, we know what genre of book we usually like to read, and our work will probably fall into that category.
So, what exactly are genres? Author Lee French puts it this way, “Literary genres are each a collection of tropes that create expectations about the media you consume.”
Genres are categories the publishing industry developed so that shoppers in bookstores can easily find what they are looking for. They’re like a display of apples at the grocery store – many baskets, each filled with a different variety of apple. I always head straight to the Cosmic Crisps.
On the display of each overarching genre, such as sci-fi, romance, mystery, etc., are baskets. Each contains a subgenre. Each subgenre is a different variety of that particular genre and features tropes that readers expect to see as fundamental aspects of the story.
But what are tropes?
Wikipedia defines literary tropes this way: The word trope has come to be used for describing commonly recurring literary and rhetorical devices motifs or clichés in creative works. 
Let’s look at the big display of sci-fi. On those shelves are many subgenres, so many that I don’t have the time or patience to list them all here.
Each subgenre includes tropes that are not usually found in other sci-fi subgenres. However, some features are common to the overarching genre, which is why they fall in the sci-fi category.
For instance, ScifiIdeas.com describes the subgenre of CyberPunk this way: Fiction relating to the science of cybernetics, which views nature as a series of interconnecting mechanical systems. Specifically, cyberpunk deals with the link between biology and computer technology, and explores humanity’s changing relationship with computer systems. Virtual reality, prosthetics, cyborgs and internet fraud are all part of the cyberpunk niche, and usually go hand-in-hand with social decline. 
For more of their article on the many different subgenres of sci-fi and the expected tropes therein, go to A Guide to Science Fiction Subgenres (scifiideas.com)
What about other genres? Fantasy is another set of shelves full of subgenres. Just as in sci-fi, each has a particular set of tropes their readers expect to see.
Let’s look at the subgenre of Portal Fantasy.
Nicola Alter at Thoughts on Fantasy has this to say about Portal Fantasy: A fantasy where characters travel from the real world into a fictional fantasy world, often through a portal or gateway. They are usually swept up in the problems and politics of the fantasy world and become important to the course of history there, then return to the real world greatly changed by their experience.
Typical Elements: Magical portals, magical objects, evil kings or queens, problematic family relationships in the real world, time discrepancy between the two worlds. 
For more of what Nicola has to say about the tropes unique to the many subgenres of fantasy, go to 17 Common Fantasy Sub-Genres | Thoughts on Fantasy.
Sometimes, we hear the comment that “certain tropes are overused.” This blanket statement is incorrect because a literary trope is a fundamental aspect of subgenre.
Therefore, one can’t say a trope is overused.
However, readers can and do eventually become bored with the books available in their favorite genres and no longer find those tropes attractive. Maybe you’re tired of Epic Fantasy’s central trope, tired of the Hero’s Journey.
In that case, it’s time to widen your reading horizons and move on to a different subgenre. Maybe you’d like a space opera.
I hear a wailing on the wind, the pathetic cries of the ghosts of bad books past. “But what if I don’t like it?”
No one says you have to finish a book you hate just because a friend said they loved it. If you’re tired of the commonalities in the books you are reading, be bold.
If you don’t try reading outside your favorite genre, you’ll never know what you are missing.
It’s like trying to get a child to eat guacamole for the first time. It’s a green paste and different looking. They don’t want to try it. But once they do taste it, they may love it. Then, keeping them out of the guacamole bowl will be a challenge.
Go out and research the many different tropes that make up the subgenres of each of the main literary genres. I have written several portal fantasies, but I also write medieval fantasy and Arthurian fantasy (two different subgenres).
If you want to know what reader to market your novel to, you need to know what the tropes are that you have written into your work. Each subgenre has a niche of avid readers, so make sure you understand what you have written.
Readers of vampire romances won’t like a story with no happy ending, so if that is what you write, you’d better have both vampires AND a traditional Romance ending.
The genre of Romance always ends with a happily ever after.
For me as a reader, if a novel is character-driven and the plot is believable, I don’t really notice the tropes. If I like a book, the labels don’t matter.
Credits and Attributions:
 Wikipedia contributors, “Trope (literature),” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Trope_(literature)&oldid=1003372690 (accessed 02 February, 2021).
 Quote from A Guide to Science Fiction Subgenres (scifiideas.com), ©2021 ScifiIdeas contributors, https://www.scifiideas.com/posts/a-guide-to-science-fiction-subgenres/, accessed 02 February 2021.
 Quote from 17 Common Fantasy Sub-Genres | Thoughts on Fantasy, © 2015/2021Nicola Alter, https://thoughtsonfantasy.com/2015/12/07/17-common-fantasy-sub-genres/, accessed 02 February 2021.
“File:Cosmic Crisp.jpg.” Wikimedia Commons, the free media repository. 24 Oct 2020, 15:25 UTC. 3 Feb 2021, 13:08 <https://commons.wikimedia.org/w/index.php?title=File:Cosmic_Crisp.jpg&oldid=499534971>.