drush – Migrating contents and config from drupal 8.7.1 to drupal 9.0.6

I need to migrate all the content and configs (entities and all of that stuff) from a drupal 8.7.1 site to a different drupal 9.0.6 site.

The point of making a new site is that I can’t update the old one to a newer version and it has been made in a… Not totally proper way. So, I need to transfer all the content to a different site that I’m making from 0, in order to replace the old one.

I’ve been investigating about it but I haven’t got any clear clue on how I should proceed or which method I could use. I don’t know even if it’s possible to make it using drush or something like that.

P.S: The old site is a multisite with 3 different sites. I suppose that’s important.

Thanks in advance 🙂