How do I retrieve all data from a FireStore document?

I am trying to access the FireStore using Kotlin and retrieve all documents from a collection called “userData”.
I think the “addOnSuccessListener” is a coroutine, but I’m using a loop in it to pass the constant “ps” and the constant "us ", assign data to “dataSets”, put the DataClass type data into an array of DataClass type “dataSets”, and " addOnSuccessListener I want to be able to use “dataSets” outside of “addOnSuccessListener” after the “addOnSuccessListener” process is complete.

package com.example.bulogapp

import android.os.Bundle
import com.example.bulogapp.databinding.ActivityRegistrationBinding

class RegistrationActivity : AppCompatActivity() {
    private lateinit var binding: ActivityRegistrationBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        binding = ActivityRegistrationBinding.inflate(layoutInflater)

        val registrationUserNameTextField = binding.registrationUserNameTextField
        val registrationSendButton = binding.registrationButton
        val backButton = binding.backButton
        var dataSet:Array<UserDataInput>

        registrationSendButton.setOnClickListener {
            val db = Firebase.firestore
            var dataSets = listOf<UserDataInput>()
            db.collection("userData").get().addOnSuccessListener { documents ->
                for (document in documents) {
                    val ps = document["pass"].toString()
                    val us = document["userName"].toString()
                    dataSets += UserDataInput(pass = ps, userName = us)

The contents of DataCLass are as follows

package com.example.bulogapp

data class UserDataInput(
    var pass:String,
    var userName:String

It is possible that I am doing it wrong. I want to eventually get all the documents from the FireStore collection, put the data in them in an array, and be able to use the data in that array after all those processes are done.
Thank you in advance for your help.