Thank you for asking, there are no stupid questions! The exchange conventions are quite well established and it can be a little tricky to get your head around them as a new person to exchanges, so it's totally understandable that you're feeling a bit confused.
You do not have to nominate to be able to participate in the exchange, but you run the risk of not seeing the characters or ships you'd like to write/draw or read about. Especially if you're interested in an obscure character or canon, it's important to get them nominated yourself rather than hope someone else will do it. It might be worth having a look at the tag set over the next few days to see if the things you want to write about are in there, and if they don't show up, then nominate them.
As for Worldbuilding Tags, we generally go by the definition of worldbuilding established by worldbuildingex: "any exploration of how a fictional world works beyond the information presented in canon. This includes (but is not limited to) more detailed extrapolation of information presented in canon, filling in gaps in the canon backstory, attempts to reconcile apparent contradictions in canon information, exploring the perspectives on canon events of characters implied to exist but not considered within the canon, etc."
So Worldbuilding tags are less about the genre of fic you're writing and more about the world you're building within that story. Worldbuilding would be like societal conventions, or laws, or geography, or the way that the world works in some way. Say you're intrigued by, IDK, how orcish society works, you might then nominate Worldbuilding: Orc Society & Conventions.
For instance, someone has nominated Queerplatonic Relationships/Romantic Friendships. In this case, this tag is saying they are interested in how these kinds of relationships work in Middle-earth. Someone else has nominated Finduilas Is Gil-galad, which is a way of saying they would like see that particular theory about Gil-galad's identity explored.
So these are things that certainly might end up in the Additional Tags on AO3 eventually. But they are very much about the world you want to see, and sort of a way of phrasing questions you might have about Middle-earth. For example, I've always been interested in trade and fashion, so I tend to nominate Trade In First Age Beleriand, and usually fashion-related tags in various eras.
I hope this helps! I wish there was a primer for fic exchanges for beginners, but please do ask any questions that come up. This is a fairly small and low-pressure exchange, so it's a good place to start.
It may help to read through some of the pages on Fanlore about exchanges such as AO3 Exchange, Gift Exchange and Exchange Etiquette and follow some of the links to sub articles about things like Dear Creator letters, Matching, and Pinch Hits. Innumerable Stars is a fairly typical AO3 exchange, though I do a combination of handmatching and automated matching. (I let the automated matchmaker set the initial matches but then check the matches for full compatibility and adjust as necessary.)
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You do not have to nominate to be able to participate in the exchange, but you run the risk of not seeing the characters or ships you'd like to write/draw or read about. Especially if you're interested in an obscure character or canon, it's important to get them nominated yourself rather than hope someone else will do it. It might be worth having a look at the tag set over the next few days to see if the things you want to write about are in there, and if they don't show up, then nominate them.
As for Worldbuilding Tags, we generally go by the definition of worldbuilding established by
So Worldbuilding tags are less about the genre of fic you're writing and more about the world you're building within that story. Worldbuilding would be like societal conventions, or laws, or geography, or the way that the world works in some way. Say you're intrigued by, IDK, how orcish society works, you might then nominate Worldbuilding: Orc Society & Conventions.
For instance, someone has nominated Queerplatonic Relationships/Romantic Friendships. In this case, this tag is saying they are interested in how these kinds of relationships work in Middle-earth. Someone else has nominated Finduilas Is Gil-galad, which is a way of saying they would like see that particular theory about Gil-galad's identity explored.
So these are things that certainly might end up in the Additional Tags on AO3 eventually. But they are very much about the world you want to see, and sort of a way of phrasing questions you might have about Middle-earth. For example, I've always been interested in trade and fashion, so I tend to nominate Trade In First Age Beleriand, and usually fashion-related tags in various eras.
I hope this helps! I wish there was a primer for fic exchanges for beginners, but please do ask any questions that come up. This is a fairly small and low-pressure exchange, so it's a good place to start.
It may help to read through some of the pages on Fanlore about exchanges such as AO3 Exchange, Gift Exchange and Exchange Etiquette and follow some of the links to sub articles about things like Dear Creator letters, Matching, and Pinch Hits. Innumerable Stars is a fairly typical AO3 exchange, though I do a combination of handmatching and automated matching. (I let the automated matchmaker set the initial matches but then check the matches for full compatibility and adjust as necessary.)