Notion is an all-in-one workspace for your notes, tasks, wikis, and databases
<aside> 👆 You can read a better summary in their own words on their website.
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Notion is designed to consolidate and replace many services like note-taking and miscellaneous writing, to-do lists, kanban boards and many more things. It's a desktop app and web-app, not unlike Evernote or your Notes app, where all the pages are arranged in a heirarchical folder-like system.
Everything you're reading here, and indeed everything on this website has been created in Notion, much like you would create new notes in Evernote, Notes, or .txt files in your folder system. As, my writing/publishing process is literally just opening up Notion, going to the page (and the many sub-pages) I have dedicated for my website, and getting to writing. Since Notion is on many platforms, I can even create/tweak things from the iPhone app, or on my iPad.
Aha! I've already answered this one, check out any of these pages: