creates a toolbar with two buttons with a C# script, without prefabs

Toolbar.cs

using UnityEngine;
using UnityEngine.Events;
using UnityEngine.EventSystems;
using UnityEngine.UI;

// creates a toolbar with two buttons with a C# script, without prefabs

// extension method to set all values of a RectOffset to the same value
public static class Extensions
{
    public static void SetAll(this RectOffset ro, int value)
    {
        ro.left += value;
        ro.right += value;
        ro.top += value;
        ro.bottom += value;
    }
}

// arranges buttons in a toolbar at the top left of the screen
public class Toolbar
{
    // initialize anchor and pivot, so that position 0,0 is top left of the parent
    private void InitTopLeftAnchor(RectTransform rt)
    {
        rt.pivot = new Vector2(0, 1);
        rt.anchorMin = new Vector2(0, 1);
        rt.anchorMax = new Vector2(0, 1);
        rt.localPosition = new Vector3(0, 0, 0);
    }

    // create a text with fixed font size and the DejaVuSans font
    private void CreateText(GameObject parent, string text)
    {
        GameObject g2 = new GameObject(text);
        g2.transform.parent = parent.transform;
        Text t = g2.AddComponent<Text>();
        t.alignment = TextAnchor.MiddleCenter;
        t.horizontalOverflow = HorizontalWrapMode.Overflow;
        t.verticalOverflow = VerticalWrapMode.Overflow;

        InitTopLeftAnchor(g2.GetComponent<RectTransform>());

        // note: GetBuiltinResource is undocumented, better use your own font
        Font font = Resources.GetBuiltinResource<Font>("Arial.ttf");
        //Font font = Resources.Load<Font>("DejaVuSans");
        t.font = font;
        t.fontSize = 20;
        t.text = text;
        t.enabled = true;
        t.color = Color.black;
    }

    // create a button with a text
    private void CreateButton(GameObject parent, string text, UnityAction action)
    {
        GameObject button = new GameObject("Button");

        button.transform.parent = parent.transform;

        button.AddComponent<RectTransform>();
        InitTopLeftAnchor(button.GetComponent<RectTransform>());

        var layout = button.AddComponent<HorizontalLayoutGroup>();
        layout.padding.SetAll(5);
        layout.childControlWidth = true;
        layout.childControlHeight = true;

        Image image = button.AddComponent<Image>();
        image.color = Color.green;

        button.AddComponent<Button>();
        button.GetComponent<Button>().onClick.AddListener(action);

        CreateText(button, text);
    }

    // create a panel with a horizontal layout
    private GameObject CreatePanel(GameObject parent)
    {
        GameObject panel = new GameObject("Panel");

        panel.transform.parent = parent.transform;

        panel.AddComponent<RectTransform>();
        InitTopLeftAnchor(panel.GetComponent<RectTransform>());

        var layout = panel.AddComponent<HorizontalLayoutGroup>();
        layout.padding.SetAll(5);
        layout.spacing = 3;
        layout.childControlWidth = true;
        layout.childControlHeight = true;
        layout.childForceExpandHeight = true;

        var fitter = panel.AddComponent<ContentSizeFitter>();
        fitter.horizontalFit = ContentSizeFitter.FitMode.PreferredSize;
        fitter.verticalFit = ContentSizeFitter.FitMode.PreferredSize;

        Image image = panel.AddComponent<Image>();
        image.color = Color.black;

        return panel;
    }

    // create the toolbar with the specified global scale
    public Toolbar(float scale)
    {
        // create a canvas with the usual components, as if you create it from the editor
        GameObject g = new GameObject("Canvas");
        if (UnityEngine.Object.FindObjectOfType<EventSystem>() == null)
        {
            // create a new event system, if it doesn't exist already
            new GameObject("EventSystem", typeof(EventSystem), typeof(StandaloneInputModule));
        }
        Canvas canvas = g.AddComponent<Canvas>();
        canvas.renderMode = RenderMode.ScreenSpaceOverlay;
        CanvasScaler cs = g.AddComponent<CanvasScaler>();
        cs.scaleFactor = 10.0f;
        cs.dynamicPixelsPerUnit = 10f;
        cs.uiScaleMode = CanvasScaler.ScaleMode.ConstantPixelSize;
        cs.scaleFactor = scale;
        GraphicRaycaster gr = g.AddComponent<GraphicRaycaster>();
        RectTransform rt = g.GetComponent<RectTransform>();
        rt.SetSizeWithCurrentAnchors(RectTransform.Axis.Horizontal, 3.0f);
        rt.SetSizeWithCurrentAnchors(RectTransform.Axis.Vertical, 3.0f);

        // create a panel with the canvas as parent
        GameObject p = CreatePanel(g);

        // add buttons to the panel
        CreateButton(p, "Test 1", () => { Debug.Log("button 1 pressed"); });
        CreateButton(p, "Test 2", () => { Debug.Log("button 2 pressed"); });
    }
}