Simple example that shows how to display an editor window on Unity startup.

WelcomeWindow.cs

using UnityEditor;
using UnityEngine;
using UnityEngine.UIElements;

// The static constructor is called before the class is used,
// but there is no guarantee when this would happen.
// InitializeOnLoad ensures that it is called when Unity loads
// or when the scripts are reloading.
[InitializeOnLoad]
public class WelcomeWindow : EditorWindow
{
    private const string AlreadyShown = "com.example.welcome_window_shown";

    static WelcomeWindow()
    {
        // The static constructor is called too early
        // in Unity lifecycle and you are limited what you can do inside it.
        // All delayed calls are invoked only once after all inspectors update,
        // so it should be safe to use to display our editor window.
        EditorApplication.delayCall += ShowOnStartup;
    }

    private static void ShowOnStartup()
    {
        // InitializeOnLoad classes are called not only on unity startup,
        // but also when the scripts are reloading.
        // This means the windows could be shown even after it is closed.
        // Keep the state whether the window is shown in session variable.
        // Session state is reset after Unity is closed.
        var isAlreadyShown = SessionState.GetBool(AlreadyShown, false);
        if (!isAlreadyShown)
        {
            ShowWelcomeWindow();
            SessionState.SetBool(AlreadyShown, true);
        }
    }

    // The rest is the code for the custom editor window.

    [MenuItem("Window/My Package/Welcome")]
    public static void ShowWelcomeWindow()
    {
        var editor = GetWindow<WelcomeWindow>();
        editor.titleContent = new GUIContent("Welcome");
    }

    public void CreateGUI()
    {
        var label = new Label("Hello Word!");
        rootVisualElement.Add(label);
    }
}