Content, terms and user/author import or migration to D8 without custom module How can I only import or simply migrate content (nodes), related terms and users/content-authors from a D7 project into a better structured D8 installation, which is similar to the old project, but meant as a project upgrade replacement, but with changes regarding how things work in D8 and which require to import the data only, apart from the code/modules of the old project?

The D8 installation is already prepared to reflect the old project with the same fields, menus, etc, but it uses different solutions for relationchips than its D7 predecessor. I would love to import content, comments, terms and users seperately (for example as CSV) and with key-ids helping to rebuild their relationchips. But ...

TBH, the migration docs I read over the last days made me even more confused than being helpful since many little in between parts and steps are not further explained so that I can not hook into and alternate the steps for my needs.

All I already know is, that Feeds and Feeds import isn't ready yet and that migrating is rather complex and is made for not only importing content but for whole project structures? But how if so many modules have no D8 version or working upgrade path?

What I read so far is:
https://www.drupal.org/docs/8/modules/migrate-source-csv/using-the-migrate-source-csv-plugin

which maybe comes close to what I maybe need for my issue. But I have a hard time to understand parts of it. So I see a use example with

 migration_groups:
  - ACME import


in it but without explanation if this line is required and what it is about. Googling for migration groups confuse me even more.

Is migrate_plus with migrate-source-csv the right direction for my purpose and is somebody out there who can help about the terminology of the migration docs and what parts are important to understand?
I have successfully migrated D7 content into a D8 site with similar content types using this small drush extension for D8 along with the built-in D8 Upgrade option:
https://www.drupal.org/project/migrate_upgrade

I believe the upgrade process from D7 or D6 is now apart of D8.3+ in the Admin UI and can simply be enabled after drupal 8 installation under the Module listing. Alternatively, you can use Drush via the module above.

Your D8 project just need localhost access to your old site files (such as images,pdfs, etc ... your old Files directory in Drupal 7/6) and database connection access.

It's a rather painless process for simple websites.

Please review the official documentation on performing a drupal 8 upgrade migration