activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <fragment
        android:id="@+id/sceneform_fragment"
        class="com.google.ar.sceneform.ux.ArFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/btn_anim"
        android:clickable="true"
        android:focusable="true"
        app:elevation="1dp"
        app:layout_constraintHorizontal_chainStyle="spread"
        app:layout_constraintHorizontal_bias="0.45"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintStart_toEndOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:srcCompat="@drawable/ic_baseline_play_arrow_24"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</androidx.constraintlayout.widget.ConstraintLayout>