obsidian

Note Management App
https://obsidian.md/
Sync
Official (commercial) sync: https://obsidian.md/sync
Alternative: Via any cloud storage (Nextcloud, onedrive, iCloud, ...), like I-MATH (https://wiki.math.uzh.ch/public/nextcloud)
Just save the Obsidian in vault in a directory which is synced via a cloud storage. 
Attention: this works fine as long as you work on one obsidian instance at a time. If you change to a different computer, always close and restart obsidian on that one.
Plugins
https://nachbelichtet.com/das-sind-meine-10-wichtigsten-obsidian-plugins