I try to use jQuery UI dialog.
Based on this, I put this in a node:
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<script>
$( function() {
$( "#dialog" ).dialog();
} );
</script>
<div id="dialog" title="Basic dialog">
<p>
This is the default dialog which is useful for displaying information. The dialog window can be moved, resized and closed with the 'x' icon.
</p>
</div>
<p>
Some stuff...
</p>
And it works! The popup window shows up automatically when the node is displayed.
But I noticed that jQuery is deprecated in Drupal 8.
So, how should I do the same thing in a more "Drupal" way?
[EDIT] Making progress...
@leymannx Using the 1rst example of Ajax Dialog Boxes page you provided, I get this working (see the screen capture).
My next step: how to have the dialog box displayed automatically when the page is reached without having to click the link?