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TL;DR: Install & activate the Classic Editor plugin.
WordPress 5.0 introduced the new Gutenberg editor as the new default editor.
In order to activate the old or classic editor by default again, go to Plugins -> Install and enter classic editor in the search bar at the top right.
The first result at the top left should be this plugin:
Click Install now in the top right of that plugin to install the plugin. After it is done installing, click Activate. After doing that, the classic editor will be active by default.