Your personal data is by default stored locally, both in the Memex browser extension and Memex Go, the mobile app. The actual location of your data will depend on your browser. For example see this Firefox forum to read about where Firefox add-ons data is stored and see this Chrome support channel to read more about where Chrome extension data is stored.

To facilitate the sync and cloud backup, your data is also stored on our Firebase servers. We do value privacy a lot, but encrypting this data is right now outside of our capacity and has strongly negative impact on our ability to iterate on the product quickly and to get it to a point of high user value. We tried with earlier iterations of the sync and backup but it was a real mess. Instead we choose the route of changing Memex business incentives to prevent data exploitation. Namely our company cannot be sold and offers only capped profits for investors and the team. We do this to remove the profit maximisation incentives usually leading to data & privacy exploitation.

Automatic backups to your local hard drive (and via your favorite cloud provider's local folder) would be completely impossible without the app. Due to the browser's security features, extensions can't automatically save things on your hard drive. Therefore it would cause a lot of clicking (= pain) to do incremental backups on your local hard drive. You would need to select a folder each time you want to make a backup.

Bookmarks made in the browser are synced to Memex, however not vice-versa. Memex saved pages will not appear in your browser bookmarks.