Backend post structure

Why are those structures different?

The two screenshots are using two different editors.

The first is using the old or Classic Editor in “Text” mode (as opposed to “Visual” mode). Note that this editor is no more the default editor and requires the installation of a plugin to activate it (which you apparently have done already.)

The second is using the new default editor, aka Gutenberg.

How can I change the structure of the new one to the old one?

Provided you’re using the Classic Editor plugin, then go to SETTINGS => WRITING in the main dashboard to configure your editor settings.