Sounds like you are looking for UI.GraphicRaycaster.Raycast.

The GraphicRaycaster is usually attached to each Canvas (otherwise you wouldn't be able to interact with anything in them).

So without referencing a specific canvas you could just go and get them all:

// Because this is dynamically changing I recommend to stick to a List instead of an array
private List<RaycastResult> _hits = new List<RaycastResult>();

private EventSystem _eventSystem;

private void Awake()
{
    //Fetch the Event System from the Scene
    _eventSystem = FindObjectOfType<EventSystem>();
}
    
private void Update()
{
    //Check if the left Mouse button is clicked
    if (Input.GetKeyDown(KeyCode.Mouse0))
    {
        //Set up the new Pointer Event
        var eventData = new PointerEventData(_eventSystem) {position = Input.mousePosition};

        //Create a list of Raycast Results
        _hits.Clear();

        // Get all existing graphicsRaycaster
        // NOTE: Of course you should rather cache this result e.g. in Awake
        // and only update it if a Canvas is added to or removed from the scene
        var graphicsRaycasters = FindObjectsOfType<GraphicRaycaster>();

        foreach(var graphicsRaycaster in graphicsRaycasters)
        {
            graphicsRaycaster.Raycast(eventData, _hits);
        }

        // Do something with the hits
        foreach (var result in _hits)
        {
            Debug.Log(result.gameObject.name, result.gameObject);
        }
    }
}

Render mode screen space overlay will always render ui in front. One way to achieve gameobject in front of ui is to change the camera render mode to screen space camera so that you can use sprite with with higher negative z value position in front of ui.

or you can use GraphicRaycaster To make a raycast tooltip list by name:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;
using TMPro;
using System.Linq;

public class UI : MonoBehaviour
{
    private GraphicRaycaster rc;
    private PointerEventData pt;
    EventSystem eventSystem;
    public TMP_Text text;
    public List<RaycastResult> results;

    private void Start()
    {
        //initialize
        rc = GetComponent<GraphicRaycaster>();
        eventSystem = GetComponent<EventSystem>();
        pt = new PointerEventData(eventSystem);
        results = new List<RaycastResult>();
    }

    private void Update()
    {
        //get screen mouse position
        pt.position = Input.mousePosition;
        results.Clear();
        rc.Raycast(pt, results);
        if (results.Count > 0)
        {
            //enable tooltip
            text.gameObject.SetActive(true);
            text.transform.position = pt.position;
            //reset text if results.count change
            text.text = "";
            foreach (RaycastResult res in results)
            {
                    text.text += res.gameObject.name + "\n";
            }
        }
        else
        {
            text.gameObject.SetActive(false);
            text.text = "";
        }
    }
}

make text mesh pro anchored on top left

and also raycast target turned off for textmesh pro

result