Add this script to your unity terrain object, and at startup it will convert terrain trees into actual scene objects. While in editor it may floode hierarchy, so by default all instantiated trees are hidden and non-selectable. You can change this behavior in the inspector.

TerrainTreesToSceneObjectsConverter.cs

using UnityEngine;
 
namespace Impostors.Utilities
{
    public class TerrainTreesToSceneObjectsConverter : MonoBehaviour
    {
        [SerializeField]
        private Terrain _terrain = default;
 
        [SerializeField]
        private Transform _treesParent = default;
 
        [SerializeField]
        private bool _convertOnAwake = true;
 
        [SerializeField]
        private HideFlags _hideFlagsForCreatedTrees = HideFlags.HideAndDontSave;
 
        void Awake()
        {
            if (_convertOnAwake)
                Convert();
        }
 
        public void Convert()
        {
            if (_treesParent == null)
                _treesParent = _terrain.transform;
            var terrainData = _terrain.terrainData;
            var prototypes = terrainData.treePrototypes;
            foreach (var treePrototype in prototypes)
            {
                treePrototype.prefab.SetActive(false);
            }
 
            var instances = terrainData.treeInstances;
 
            var terrainSize = _terrain.terrainData.size;
 
            for (int i = 0; i < instances.Length; i++)
            {
                var instance = instances[i];
                var prefab = prototypes[instance.prototypeIndex].prefab;
                var rotation = Quaternion.Euler(0, instance.rotation * Mathf.Rad2Deg, 0);
                var position = instance.position;
                position.x *= terrainSize.x;
                position.y *= terrainSize.y;
                position.z *= terrainSize.z;
                var tree = Instantiate(prefab, position, rotation, _treesParent).transform;
                var scale = tree.localScale;
                scale.x *= instance.widthScale;
                scale.y *= instance.heightScale;
                scale.z *= instance.widthScale;
                tree.localScale = scale;
                tree.GetComponent<ImpostorLODGroup>().RecalculateBounds();
                tree.gameObject.SetActive(true);
                tree.gameObject.hideFlags = _hideFlagsForCreatedTrees;
            }
 
            foreach (var treePrototype in prototypes)
            {
                treePrototype.prefab.SetActive(true);
            }
 
            _terrain.treeDistance = 0;
            _terrain.treeBillboardDistance = float.MaxValue;
            _terrain.treeMaximumFullLODCount = 0;
        }
    }
}