The best and most popular apps for building a second brain includes:
Notion: a popular all-in-one workspace that allows you to create databases, take notes, and organize information in a streamlined way.
Evernote: a note-taking app that allows you to capture, organize, and share information across different devices.
Obsidian: a note-taking app that focuses on building a personal knowledge base through the use of markdown and backlinking.
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Starting with Notion template is the easiest option, in my opinion.