Readia's answer
Readia focuss on "Retain what you read." A user can take an exam from his Kindle highlights and set a schedule for reviewing.
Readwise - Effortlessly rediscover and organize your Kindle highlights
Booky - Online bookmark manager. Store your favorite bookmarks online. Available everywhere. Private. Customizable. Fast.
Clippings.io - Organize the notes you make on your Kindle
Klib - Kindle & iBooks Highlights Manager
Tressel.xyz - Save content from the Internet (like Twitter & Reddit) and export it to Notion, Obsidian & Evernote
Newsletters to Kindle - Read your newsletters on your Kindle