Shelley, quarantine love stories! Who knew. A play on the “forced proximity” trope with friends-to-lovers thrown in.
Your terminology troubles are completely understandable; as with most things editorial, no one really agrees on anything. So, for our purposes I would say a storyline is just the plot–whatever happens in the story that winds up with the two MCs in their HEA. A trope is just a type of storyline that has been used a lot, and that readers tend to enjoy encountering.
A category is just a big general marker for the type of content one can expect: “young adult” will have a young adult protagonist; LGBTQ+ will have a protagonist who is LGBTQ or +.
We could call “romance” itself a category but since almost everyone calls it a genre, we’ll call it a genre, too. Therefore, specific types of romances are called subgenres. They fit in the overall category of the genre romance but have specific characteristics not in common with all romances. So an historical romance is a subgenre of romance.
Category > genre > subgenre.
A YA novel can be a romance but doesn’t have to be; a romance can have historical elements but doesn’t have to. Hope that helps a little?