How Do I Query current ID against ID's in a collection to show only documents with similar fields and value using react Js and Firestore

I have two documents, currently I can view the documents independently and the respective ID's can also be seen in my web address. So what I'm trying to achieve is to show document from my firebase collection that has a similar field with similar value; for instance if one of the Id(f3aNQYFyv9hpg0ZLikWt) has a field(regNo) and another Id(Ta0yfjgbq9B30OLTnPzx) with a field(regNo) both having the same value(regNo: "AFY/2013/6440"). Since I can pull the current Id with useParams from the web address, I want to compare it with the documents in the collection to check if there's another Id having similar value from a field...
So currently I've pulled all document from the collection:-

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<!-- language: lang-js -->

    const [data, setData] = useState([]);

 useEffect(() =>{
    // LISTEN (REALTIME)
    const collectionRef = collection(db, "dentist");
    const q = query(collectionRef, orderBy("timeStamp", "asc"))
    const unsub = onSnapshot(
      q,
      (snapShot) => {
        let list = [];
        snapShot.docs.forEach((doc) => {
          list = [{ id: doc.id, ...doc.data() }, ...list]
         
        });
        setData(list);
      },
      (error) => {
        console.log(error);
      }
    );
    
    return () => {
      unsub();
    };
    }, []); 

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What I tried to do was compare the current Id against all Id's in the collection to check which of the documents has the same value for the field(regNo). I had to first check if I'm getting data containing those similar fields with value, which worked, here's the code:-

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   const [data, setData] = useState([]);

    useEffect(()=>{
        const getList = async () => {
            const collectionRef = collection(db, "dentist")
          const q = query(collectionRef, where("regNo", "==", "AFY/2013/6440"))
          await getDocs(q).then((task)=>{
           let medData = task.docs.map((doc) => ({...doc.data(),
          id: doc.id}))
          setData(medData)
          console.log("medData", medData)
          }).catch((err) =>{
            console.log(err)
          })
        }
        getList()
      }, [])

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This actually returned every document with that particular regNo but I wanted it to compare the current ID against all the Id's in the collection, which didn't work plus I don't want to hardcode the regNo value, because If I'm to compare the current Id in my web address with the others in my collection, it should loop through to see which documents share the same value for the field(regNo)..
This was what I eventually added that returned an error

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<!-- language: lang-js -->

   const [data, setData] = useState([]);

    const {userId}  = useParams();
    let id = userId

    
    useEffect(()=>{
        const getList = async (id) => {
            const collectionRef = collection(db,`dentist/${id}`)
          const q = query(collectionRef, where("regNo", "==", "check for the same value of regNo(This is a what I want it to do)"))
          await getDocs(q).then((task)=>{
           let medData = task.docs.map((doc) => ({...doc.data(),
          id: doc.id}))
          setData(medData)
          console.log("medData", medData)
          }).catch((err) =>{
            console.log(err)
          })
        }
        getList()
      }, [id])

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Defining a CollectionReference with const collectionRef = collection(db,`dentist/${id}`) cannot work because there is an even number of path segments. You must have an odd number of path segments, e.g. collId or collId/docId/suCollId or or collId/docId/suCollId/subDocId/subSubCollId, etc...

So, if in a collection you know the ID of a document with a given value for regNo and want to get all its siblings documents in the collection you need to query the entire collection with a where clause on regNo.

In other words you need to:

1. Fetch the first document, based on its ID, in order to get its regNo Value
2. Use this value to query the entire collection
3. Identify all the docs in the query results that are not the first document (to be done with a loop in the front end, based on the initial document ID)