If you use the Builder to create the tabs and custom fields, you can drag and drop to re-order the tabs and fields.
If you are using the code to register the tabs and custom fields, just move a tab above another to re-order them.
Unfortunately, the Builder does not support re-ordering the tabs on the settings page. You can use the tab field in the Field Group to re-order them. I will inform the development team to consider supporting this option in future updates.
I understand now! And it is better, much better, to use Tabs inside Custom Field instead of using it in the page settings. I have less Custom Fields 'groups' now.
Could I easily merge different Custom Field Groups? Remove the group and add the same id in another Group? Maybe there is a better of faster way.
Yes, you can use the builder to export field groups to JSON files, edit the file to add custom fields, and re-import to the builder. But please be careful with this way, it will break your field group if there is a small error like a typo.