The latest updates to WP1 have brought new themes and more changes to the interface. The views in the editor are aiming to make writing a more focused and calm experience. The usual left-side panel offering an overview of admin function is hidden by default now.
If you are very new to working with WordPress, you might like this as a distraction-free way to write your text. However, if you have worked with WP before, then this might catch you out and be quite confusing.
This is when plugins are useful ~ restoring a previous default. To stop the automatic full screen mode, install and active the following plugin:

Disable Block Editor FullScreen mode
By Ankit Panchal
Once installed, you’ll see the admin interface remain consistent throughout. You will still be able to manually set your preferences if you wish.
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