You can switch between the two modes by clicking the tabs in the top right corner of the editor: “Visual” and “Text”. When in the visual mode, you can also enter the “Distraction-free mode” by clicking the fullscreen button beneath the tabs. When in that mode, everything but the post editor is hidden, allowing you to focus on your writing.
With the release of WordPress 5.0, the old post editor was replaced with the block editor, which constitutes the biggest change to the WordPress editing experience since the system was launched. The block editor allows you to construct the content of your posts and pages by building it with blocks. Blocks can contain headings, paragraphs of text, images, quotes, and so on. Blocks can easily be added, removed and edited to create more engaging page layouts, and custom blocks can be added by themes and plugins.