Drupal 7 field groups defined in Features field_group.inc files will not migrate.

Wow, I was so so so so confused as to why four content types' field groups had migrations and no others! (because, Features were maintained for a while after this site was built but not forever, so there are content types and stuff with outdated or zero features)

Thank you everyone on this thread for sharing, and thank you @kevinquillen for the solution share! -- I haven't tried that part out yet, we're in a discovery phase, but I'm adding this to my notes for when we get to implementation.

I've used the above trick a few times since then, YMMV.

@kevinquillen What's the reason for putting the features modules into the Drupal 9 site codebase? I'm missing where/how that part fits in.

OH! Never mind, I see it now!!

They only exist so PHP can read the definitions of them in, in order to create the database records needed for the migration to pick them up.

Thank you! -- in case it helps anyone else who stops by this thread, my process for using the generated migrations was (and idk if this is best / correct, but it ended up working for me):

TL;DR: Run the field group migrations after field, field_instance, and field_instance_widget_settings migrations.

More specifically, this was my order of things:

1. Run some migrations that tend to be dependencies of node/content migrations:
   
   1. upgrade_d7_user_role
   2. upgrade_d7_filter_format
   3. upgrade_d7_node_type
   4. upgrade_d7_user
   5. upgrade_d7_field
   6. upgrade_d7_view_modes
   7. upgrade_d7_taxonomy_vocabulary
   8. (then specific taxonomy/vocab migrations)
2. Then...
   
   1. upgrade_d7_field_instance
   2. upgrade_d7_field_instance_widget_settings
3. Then, "field group" migrations, i.e. upgrade_d7_field_group_node_news
4. P.S. It also worked when I ran the field group migrations before the field instance widget settings migrations (accidentally, but it worked).

I came across a couple other "site building things" that don't migrate smoothly / out-of-the box due to not being in the source site database when they're in Features (image styles, views), and posted about it here:
https://www.drupal.org/forum/support/upgrading-drupal/2024-01-24/upgradi... →

Mainly, I'm just sharing in case it's helpful to others, but if anyone's run into other "site building things" that behave like that, I hope you'll comment on the Forum thread!