It is important to understand that upgrading Drupal 7 or 8 to Drupal 9 requires significant site rebuilding. While existing content mapping and Node infrastructure can be used to speed up the process.
The frontend code as well as the underlying Drupal functionality/theming must be rebuilt from scratch in order to make it compatible with the latest technologies and take advantage of new features. You can know more about the drupal migration services via 95 Visual accordingly.
To be more modern and scalable, the management, theming, and templating have been completely reworked. Twig replaces PHPTemplate as the default templating engine. Composer can now be used to add modules instead of Drush.
Many contributed modules have been incorporated into Drupal core. Although these are welcome improvements to make Drupal more modern and manageable, they require substantial development. Frontend templates will need to be rebuilt as part of the transition to the templating engine.
This means that frontend designs will have to be recorded and then theming onto Drupal. It makes sense to combine the project with a redesign if you have been thinking about it. This is because the frontend recording needs to be done with new designs in the future.
Why pay for (and go through) the frontend process again? You don't have to go through the frontend process all over again after a redesign. Instead, you can pay once and get the design that you want.
We recommend creating a custom migration configuration using the Migration module to move the content from your Drupal 7 content database to the Drupal 9 content database. To maintain control over the layout of your pages, you will need to use Paragraphs if you are using it to manage Drupal 7 and Drupal 8 content.