How can i make relation between two tables?

Here there is no problem with data duplication, you can use a common id in both tables and query each one with the common information or simply put the information you require from the other table in a single table.

You can simple add relations by adding reference to that documents example code in JavaScript: const sendContactUs = async (userData, uid) => { await firestore().collection("ContactUs").add({ Email: userData.Email, phoneNumber: userData.phoneNumber, message: userData.message, uid: uid, user: firestore().collection("Users").doc(uid), status:"Pending" }) }

Firebase in not relational database. Its a NoSQL DB like MongoDB. So theoretically you can't make relation between tables. What you can do is keep id reference of one table (or better say json file/data) into other and query accordingly or filter it locally on client side with high level functions like reduce and/or filter.